You can figure out pretty quickly whether a founder has been forced to actually sell something or whether they've been living in pitch mode. And the difference shows up in a really specific way.
Ask them about their customers. The founders who had to sell to survive talk about specific people. They know the person's name, what their actual objection was, why the third conversation went differently than the first two, what made them finally sign. The detail is almost uncomfortable sometimes.
The pitch mode founders talk about personas. "Our customer is a mid-market ops manager who struggles with workflow inefficiency." Technically an answer. Completely useless information that tells you they've been reading about their customer rather than talking to them.
The brutal thing is that early fundraising success can actually mask this for a long time. A founder who raises a seed round before they've had to sell anything gets 18 months of runway to stay in pitch mode. They raise the seed talking about potential. They raise the A talking about momentum. By the time you're at the B someone finally asks a hard question and there's nothing underneath.
The founders who couldn't raise early and had to go find a paying customer instead got the most important feedback the startup world has. Someone who has no obligation to be nice to you decided your thing was worth money. That's it. That's the whole game early on and most of the ecosystem is set up to let founders avoid finding that out for as long as possible.