It really depends on the ICO, you need to really look into each, read as much as possible, check out other peoples opinions, see if others have invested in it to check if it is a scam or not and so on... Once you actually feel like the project has potential and it can really have a good use. It is time to invest.
I usually see comments or suggestions given by others, because they are usually really doing better research (if you are lazy in this case), because other people's comments I think also determine whether the project is a scam or not