If I got an LCD, and I didn't know anything about it, and there were no markings on the pins, how would I go about interfacing with it? For example, I got this small (around 2.5") colour display out of an old printer, I tried everything I could, looking up a datasheet, asking on forums, but I was unable to get it to work. What is a general process I could use? Is there like, a few common interfaces I could use?
Your best shot is to hook up a signal analyzer on the LCD interface while you still have the working device it comes from. Then you can look at the captured signals and try to identify which protocol it is, what the init sequence looks like and what are typical displaying commands. If all you have is the LCD, there's noting you can analyze, only guess.
Another creative approach I can think of is to take a few pictures of your LCD and p̵o̵s̵t̵ ̵t̵h̵e̵m̵ ̵h̵e̵r̵e̵ use reverse image search. If your LCD is widely used, you will usually find out its model name. If not, I wouldn't even bother, since you can get a known LCD for as little as $2 from e-bay or similar.