It's okay if they change core business. The problem is when employees do what they didn't sign up for, or when they focus on things that are not the core business (e.g. consulting or some other company held by the same owner).
I should have started looking for jobs when my hours were cut from full time to part time at a startup I was at. I guess I was expecting a rebound to happen.
A few months later, I messaged the founder asking if he had an ETA for when the work hours would go back to normal. I got laid off after that.
What is a startup without a product and sales ? Or do you mean someone internally can use the product but it is still not out for more then a month ? Yes, a sign of failure.
There's a thousand signs, you will always see some. It's not one thing in particular, it's a combination of things. It's the ability to recognize that more and more of those things are happening instead of less and less.
It might be subtle and is similar to what others have said, but if you find the founders running out of steam, losing passion, for the startup, that can be a sign.
