VC or investor check. Include standard items 1. Did you quit your job; 2. Did your cofounder quit his job; 3. is your sales validated.
A thorough validation of everything you said.
If you say "I have quit my job', show them the severance pay check or letter accepting your quit, and then they will still call your old employer.
If you say "so and so in a big company is very interested because mine is better than the competition" they will ask the phone numbers of both so and so AND the competition.