Assuming you just can't get any traction (more than 10,000 users) after a year, would you give up?
Might want the check out "The Dip: Knowing when to Quit and When to Stick" by Seth Godin...
One short measure you can decide on is uptake. If there is little or no uptake you know you are in trouble. But I agree there is a lack of information. Startups are designed to stumble all the time. Is keiretsus` startup stumbling? Is the lack of a compelling idea holding it back? Is the product something that "customers want"?
Is it an Artix? ~ http://www.paulgraham.com/bronze.html
Death of a startup is due to cumulative losses, not one factor. If you'd like to build a better scenario I'll throw some change in. ;)