Power Supply or MoBoard?

greetings to all who follow this.

It's my turn to ask… usually I'm on the answering side.

Read the existing answers but I may have missed something.

iMac was up, operating normally, then suddenly out. Black. No warnings.

LED 1 is on when plugged in… trickle juice is trickling.

Depress power button and all LEDs go out immediately. Nothing else.

It seems like it could be the power supply but is there any other way to prove it before rolling up the sleeves and opening wallet?

thnx

<<<z>>>