Well, ask for proof if you are uncertain of their claims.

Also, don't just join a startup due to it being backed by a famous VC. If you can't get behind their product, goals, atmosphere etc then you will have a miserable time regardless.

To really get a feel for the co-founders, you need to spend some time in non-interview situations to fully understand their character and traits. Maybe try and get them to sit down for a coffee and put your concerns on the table. Good co-founders should appreciate the transparency.