Ask a question the founder can actually use
Most questions under a pitch are polite opinions in disguise. A useful question points at something the founder cannot see from the inside.
Start with what you did not understand
The most valuable thing you have is that you are not the founder. Something in the pitch was unclear, skipped, or assumed. That gap is invisible to the person who built it, and naming it precisely is more useful than any praise. "I lost you at the part where the data comes from" tells a founder exactly which sentence to fix.
Trade opinions for specifics
"Have you thought about competitors?" is not a question, it is a verdict. "Who do you lose deals to today, and why?" is a question, and the answer teaches everyone reading. The difference is that a good question has a concrete answer the founder either knows or should find out.
If you have relevant experience, say so briefly and then ask. "I ran ops at a clinic; how do you get this past the front desk?" is worth ten generic comments.
Questions that usually help
Who used this last week, and what did they do with it? What happens to a customer on the day they stop using it? What did you cut from this pitch that you wish you had kept? What would make you abandon the idea? Which part of the demo was real, and which part is still a plan?
Questions that rarely help
Anything the founder cannot answer without a crystal ball: how big the market will be, what a large company might do, whether it will work. Anything you could have looked up yourself. Anything that is really a suggestion for a different product. Save those for the comment box, and label them as opinions.
Finally, one question at a time. A list of five gets a vague answer to all of them.