Hi,

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.