Given that I've answered more ID questions than anyone else on Bio.SE to this point, I suppose I should make a comment.
*grumble grumble* revealing trade secrets... *grumble grumble* :p.
Many common species (especially cosmopolitan ones) get asked a lot. Many of us know these species well. Otherwise...No rock is left unturned!! If there's an available online or on-the-shelf source, I put it to use when necessary. This only works because I'm familiar enough with common species (especially pests) and characteristics that indicate to me higher taxonomic ranks to narrow my choices. I'm also used to gravitating back to useful ID tools (specific keys, websites, etc.) that I've previously found to be useful.
- many times these tools/sources are quite nuanced, and so are not worth listing here in any capacity (but check my numerous answers for examples!).
Let me explain:
EXPERIENCE: I definitely have a good understanding of most major phyla and often can ID something to a specific order/family, so I can usually look at something and instantly begin narrowing my options. In other words, since I've been IDing for a while, I often have an idea about what I'm looking at and know how to start narrowing my search. (Research experience in plants, insects, herps, mammals, and natural history as well as my experience as a bio professor help me out a lot, too, in recognizing good starting points for searching :p)
- I'll also use other people's comments/posts to aid my jumping off point if it's something I'm less familiar with (e.g., FUNGI) or nuanced/endemic type of species from the other side of the world.
INFORMED SEARCHES: after narrowing taxonomically, my search terms get more specific. When I don't know exactly what I'm looking at, it's all about knowing the right vocab, A&P, habitat, location, or other information to pursue given whatever taxonomic or location info I do know or can discern. So instead of searching "round green bug" for example, I might search "green hemipteran nymph." If I know the location of the specimen, I can, for example, additionally narrow to: "green hemipteran nymph Wisconsin."
Ultimately, this is why a good image and knowing a specimen's location and size are SO KEY to narrowing down to the right answer. I can modify my search terms based on relevant physical, behavioral and location info! (this is why lacking any of this info usually leads to downvoting or incorrect IDs).
- Sometimes, I can ID something else in the picture that gives me hints for what else to search or look for. For example, maybe the OP's green insect is on an apple leaf in his/her picture, and so I can narrow my search or hone in on other specimens that hang out on apple leaves, too. Or maybe I notice a tree fern in the background, which narrows my geographic options significantly.
Being able to perform informed searches comes from repeated experiences, seeing similar taxa, and -- truthfully - patient trial and error.
- Sometimes, it requires reading additional sources to become familiarized with what characteristics are important and what to be looking for. I certainly have perused a 30 year-old technical note describing the reproductive parts of a rare family of insects on the other side of the world just to know where to take my search next...
Searching using relevant key words and being able to recognize seemingly-unrelated species that come up as actually being related to the specimen in question is key, too. Novice IDers are looking for exact replicas in a google image search -- good luck :p!! Recognizing that species that come up in image searches are related to my target specimen just based on apparent similarities is key to allowing me to follow those "rabbit holes" to gain more info and inch my way to an answer.
Using my green insect example above, perhaps a google image result or field guide shows me an orange hemipteran nymph also from Wisconsin. Chances are, following that line of evidence might teach me something useful.
This line of inquiry is especially useful for IDing specimens in geographic places I know nothing about or have never visited. Sometimes, you need to invest the time to learn an area's language, nuances, potential species lists, etc., to make a breakthrough in IDing your target specimen.
- Sometimes, changing the language settings in your Google search to a language spoken in the target region will drastically change your search results...
When available, I use online or on-the-shelf keys, field guides, journal articles, textbooks, etc. as these are ideal for IDing species. This is most easily done when someone asks about specimens in my region of the world, because I can quickly pull a guide I own off the shelf or simply react to having previously seen the organism myself.
The goal is to always check as many reputable sources I have at my disposal to make sure I'm not giving a false ID!!
I always double check sizes, physical characteristics, known ranges, behaviors, etc. with whatever info the OP provides in their question (again why asking detailed questions counts!). If something doesn't match well enough, I now have an almost certainly related species to use to start my next narrowed search....
Inherent in this process is coming to recognize which sources are not reputable and therefore avoiding letting such poor-quality resources mislead me. (Hint: just because an image on Google search says it's one species, doesn't mean it actually is that species -- e.g., search "bedbugs" sometime to see Google utterly fail!). Image aggregation sites like Imgur, Pinterest, etc. also are notorious for having mislabeled images, and I'd also be cautious of using Q&A sites or other community identification sites as anything but initial jumping off points -- always corroborate with more reputable sources!
As an aside: I will often try to duplicate my findings from esoteric, print-only, or pay-wall sources with other reputable sources that are freely accessible by the general public (typically digitally). The goal is to provide adequate support without my posts simply citing a bunch of page numbers in hard-to-find sources. I want others to learn more about the specimens in question, so I try to provide a trustworthy roadmap for them. (Often this means linking to wikipedia pages or university extension websites).
Finally, I'm always explicitly clear about my own shortcomings in IDing. If the OP doesn't provide adequate info, or if I'm just uncertain about the specific ID or uncertain about ruling out similar options I know little or nothing about, I try to never make definitive claims.
Often I'll say something like "If this, this, and this.., then you likely have a..." or "looks like...", etc.
The worst is when someone speaks using terms of certainty or authority and they're wrong. I don't want to add to the noise of the internet.
This is why I ALWAYS PROVIDE REPUTABLE SOURCES TO BACK UP MY CLAIMS (or, sometimes, just to act as bread crumbs so you know what process I went through to get to my answer).
See my answer for Plant identification methods, a post over at Outdoor Stack Exchange, to see an explanation of what an ID process may look like.
- Don't stop at the "close-enough" image. Do your homework. Get the right answer. And provide support for why your ID is likely correct!