Warning Signs That You Might Not Want to Bid on That Job...

If an employer has only paid 80% of their billed invoices... You might not want to bid on that.
Why? This employer might have valid reasons for not paying one bill, but if 1 in 5 of the invoices they receive go unpaid it is probably a sign of financial difficulty, unclear project instructions, or a generally disagreeable employer.

If a job post says, "Gets excited about the possibility of growing with a startup."... You might not want to bid on that. Why? This is code for "We are going to pay you practically nothing." This is like playing the lottery with your income. This is a new business and like any new business it plans to grow into a big business. It is being upfront in the fact that it cannot pay you well for your work, but it is being dishonest (intentionally or not) by implying that you might make a lot of money if you take a chance on them. If a business starts out paying you peanuts, what reason would they have to increase your pay even if they could afford it later? Yes, I am sure one in a million would, but a more likely scenario is that they will pay you the same thing forever. This has actually been a complaint on some freelancer boards- they don't understand why freelance employers don't feel they need to pay more after working with freelancers long-term. 

If an employer has a positive rating below 90% on Guru... You might not want to bid on that.
Why? It used to be that Guru allowed you to dismiss 10% of your negative feedback. It no longer does this and 100% positive ratings have (naturally) been plummeting. 10% is a good amount to prevent freelancers and employers from using feedback for blackmail. I would say in my lifetime I have deleted the feedback of 3 employers. Considering I have worked for at least 100, that is not too bad odds. Without the feedback deletion, the same rules should be applied: 90% (and above) positive feedback is good. The flip side is that once it falls below this (and the freelancer or employer is already established with more than one or two paid jobs), there is something wrong with the way the employer deals with freelancers. If you work with this employer, chances are good you will have problems because not every freelancer leaves feedback.

If an employer has not paid out any money or hired any freelancers... You might not want to bid on that. Why? Yes, everyone is new, and of course I bid on jobs posted by new and old employers. I still felt I needed to include this as a negative reason to bid because the chances of a new employer hiring anyone are much less than the chances of an old employer hiring someone. New employers frequently don't know how much to pay for their project. When they find out that quality freelancers charge more money than what they expected and often take more time than what they expected, they simply don't hire. They are also intimidated when they get 100 bids on their project--many from people who have no business bidding. Receiving "canned" bids also is a turn-off (but this shouldn't be in most cases).