How can you test the power supply

My iMac doesn't power up. I watched a video on how to change the power supply but how can I check to see if the power supply is or is not working properly? Could my iMac have another issue as to why it is not powering up? It seems like there isn't any power getting to the computer at all. I checked the power cord and there is power going through the actual power cord from the outlet. I also checked to make sure there was power coming through the line that connects to the power supply (the 2 prong connection). Could it be the power button itself? And can the power button be replaced?

I don't see any LED lights inside my iMac. I've looked for them.