For that you have to gain a lot of knowledge about icos. And this days scam icos are very smart so sometimes its hard to discover one. But still if you really want to know if a ico is scam or not the first thing you have to do is read the white paper. It is the main thing in a ico and you have to check it up again and again. You have to read it and understand it how technically it is written. Then you gave to check the website and talk to community and founders. Of you find they are find also then go for how relucent the project is now and will be ain upcoming future that's it. Still sometimes its hard to catch one.