Founders no longer work on it full time. This is an extremely strong indicator and I've never seen a startup recover from this.<p>Employees get tasked to things other than the core business of the startup. Usually consulting or some other side business.<p>Startups rarely ever die loudly. They just seem to crawl off away from everything else and try to die as quietly as possible.
Things I have seen that are usually indicators (although not always as it can vary from company to company).<p>* Hiring comes to a halt.<p>* Senior members begin quitting.<p>* Transparency becomes infrequent.<p>* Teams are being dismantled.
I dont know who posted it originally (here on HN) but there are a lot of signs which may be true for startups as well as for non-startups:<p><a href="http://wiki.c2.com/?WarningSignsOfCorporateDoom" rel="nofollow">http://wiki.c2.com/?WarningSignsOfCorporateDoom</a>
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.