When talking to startups, a question I love to ask is what are you not working on.
The basic premise is Startups have limited resources, so in many ways what are you consciously not working on is as important as what you are working on. While not indicative on it's own, if a early stage startup tells me they're working on say AWS, and Google, and Azure, and on-premises, it creates an area to probe further that they might not be careful about selecting the right thing to work on next which does become a red flag.
The basic premise is Startups have limited resources, so in many ways what are you consciously not working on is as important as what you are working on. While not indicative on it's own, if a early stage startup tells me they're working on say AWS, and Google, and Azure, and on-premises, it creates an area to probe further that they might not be careful about selecting the right thing to work on next which does become a red flag.