There are things I/we need to consider first in order for us to trust a new altcoin but first want to let you know that even though you've met this requirements there's no guarantee that the new altcoin is trustworthy because some of the altcoins that is trustworthy becomes scam at the end so the things is that you really need to face a high risk whenever you're going to invest in altcoin, so keep that in mind. So here are the things I/we consider for us to trust a new altcoin.
1. Background of the team (If the team didn't give info about themselves then it's a little bit shady, but it doesn't mean that it is a scam... but there's a high chance that it is a scam)
2. Professionalism of their works such as, their official website, ann thread, social media accounts, etc.
3. Read their whitepaper and look if it's gonna happen or is it a good concept or just a promising words.
4. Check the roadmap if and other technical details
5. Do your own research.
6. Find an active and thriving community.
7. Look for special proof and validation.
8. Trust your guts.
So here are the things I consider before I trust the altcoin but it doesn't mean that it's 100% legit and trustworthy. Check this links to avoid ICO scams and how to avoid it.
https://www.entrepreneur.com/article/306563https://www.cryptocompare.com/coins/guides/how-to-avoid-scam-icos/