Use Archivd. :)
It depends on the situation, how early the company is, how tight the sub-community, etc. But there are 5 major things to check.
a) Funding? See if they are listed on the angel/VC website. Check news reports, press releases, etc. See what other companies the investors have funded and their outcomes.
b) Personnel? See if there are any names in early reports that no longer appear. Make a note of the business / manager / technical mix. Also check to see if the people have worked together before.
c) Competitors? Do your own deepdive for plausible competitors and make a note of their funding, team, and possible business models. Take care to find dead competitors, ie foo.com is cool but Bar Corp already failed at a similar model. Why? How are you different?
d) Relays? You might know someone who knows someone who knows someone at the company. Get their opinions of the people and the business.
e) Demo? Ask for access to their prototype. If Arrington has an invite, then potential hires should too. They might get cagey, might not, either way it's a good data point.