iMac screen black power supply or motherboard issue

[susu]

Imac screen black no chime I don't know if is a motherboard or power supply issue, led 1 and led 2 are on, not recognize usb keyboard or mouse keyboard, fan are spin, not recognize bluethoot, not eject cd. No boot from cd.

 

[HacKage]

if there is no LED3, that points to video issue so will either be video card or logic board

 

[evildead1971]

if there is no LED3, that points to video issue so will either be video card or logic board

how do i check to see if the LEDs are on or off?