My own take elaborating on Joe Strazzere and O. Jones' answers. This requires a conversation where you get answers as to the company's funding, runway and progress towards the market and sales revenue.
Key factors for startups are runway (how many months they've got funding for) and sales (which helps towards profitability, and/or enables further funding rounds).
It's a very reasonable area to ask about, since every investor will. Any business should be able to give you clear answers to high-level questions in this area.
Note that their answers may be anything from realistic, to possible, to overstated, to wildly unrealistic, to incompetent or fraudulent. You should try and quietly assess for yourself the quality of their product, plans & organization.
Some questions I would ask:
"Please tell me a bit more about your company.
"How are you funded for payroll and operating expenses? Please tell me a bit about your investors. How long is it until you need another round of financing?"
"Why do you believe your product is compelling?"
"Where are you in the go-to-market plan? Have you found a good product-market fit? When do you expect to reach profitability?" listening for whether they have customers & sales yet
"How long is your runway to launch this product?"
"What are the critical factors for the business to succeed? How can I help?"
Personally I would not recommend to dig directly for details of customers and competitors, this seems perhaps a tiny bit commercially sensitive. I would try and assess and understand the value & marketability of their proposed product, and their capability to build and market it successfully.
Assuming the business proposition is somewhat plausible, the normal failure cause is for funding to run out before product-market fit can be found or profitability can be reached. These questions are designed to help you answer that.