What are symptoms of a bad power supply?
The fans and hard drive power up, but no display and no beeps.
According to you, there are no beeps, the fans work, the HDD works, so presumably the system boots up OK. It's just that you have no display.
So why do you suspect the power supply?
If your monitor is working OK, it should at least indicate a no signal condition if there is no display coming from the desktop. Check the monitor's onboard menus to at least prove that the monitor does have some functions working, e.g. the screen works
Can you try another monitor?
To answer your question though. Here is a link to the user manual for your desktop. Scroll to p.79-80 to view what the different power light conditions tell you about the state of the power supply in the computer.
Sounds like the system is not completing POST. on dell machines there are a sequence of numbers somewhere on the case that indicated its startup phase. If it gets stuck at a certain spot then something is not letting it complete POST. The power supply is not always the cause of the problem. If you have installed any new hardware or if anything has happened to the computer recently then that might be the cause.