Me, im taking a research about the people behind the project. If they are reputable developers. I consider them as legit project. If i found out that they are not real. That's absolutely a scam. If you are tech savvy or code specialist. You can check their source code to github and compare them to other altcoins. Im not good at codes. So im not doing this. Check the whitepaper. Some project are just copy-pasted from other projects. And finally, the roadmap. Check the project if they have roadmap. Roadmap is their aim to accomplish their goals by presenting whether short or long term milestones. That's my strategy to determine what is scam or legit project. Good luck.
What this user said is good advice. I would also say that you should see if they either have a Facebook profile or a Linkedin page with a phone number to see if they are real people and see if they are planning on attending any big events or conferences. Do as much research as you possibly can. And never invest more than you can afford to lose.