Personally, I try and look at it purely from a Product perspective:
- How unique is the proposed product
- How is the team running the product
- How close to having a live beta version of the product available
- How frequent will large announcements take (once a year or monthly)

The community plays a huge part for the Alts - for example Deep Onion, the community they have is exceptional and it promotes a common consensus among investors to hold and promote their asset. This builds a healthy investment ecosystem in which Alt coins should thrive.
I also like to act on gut - some Alt's catch my eye due to their technological potential alone - see ADA for example - I see purely undeniable potential