Read their whitepaper. Check their developers. Ask questions about their token and its primary purpose. Though none of these might totally let you know if the coin is a scam or not as at first none of them are scam, but you will have the slightest idea if the project is worth investing in or not if you check these things before. One more sign that I see a lot of times that makes it suspicious is the promotional rewards are way higher then how much they should be. It probably means they are trying to attract more people to advertise their service and let the coin get some value and then they might take the money collected from ICO and run away with it.
Interacting with their team is probably the best way to decide whether a project is legit. Ditto on at the whitepaper, roadmap and bogus airdrops. Most of the time if a coin is airdropping a huge number of coins, it just shows that they don't value their own coin very much. If they did they wouldn't be throwing handfuls of it away...