Any way to tell apart a CPU defect from a mainboard defect?

From Motherboard, CPU and RAM Troubleshooting, if you do not get any beeps but can hear the fans, then it is probably a motherboard failure (but do read this on-line book entirely). Some of the tests require having some spare-parts on hand, so can be better done by a tech-shop.

See also Laptop CPU Failure Analysis and Solution can be helpful with its discussion of "CPU rust". (Link is dead.)

Connect an internal PC speaker to the mainboard an listen if there is a 'code'. Look for it in the mainboard's manual

I have the same problem, and it's a real head scratcher. I removed the main board and noticed
some discoloration on the back near the CPU (which led me to believe the motherboard was to blame), but was still not 100% sure, so I removed all the RAM and powered up the board expecting to hear some beep codes, there were no beep codes.

If the mobo was functioning correctly I should have heard the beep codes indicating a problem with the RAM but still no codes.

I replaced the motherboard and thankfully it was to blame.

Before you go rushing out to replace your motherboard, make sure you give your CPU a proper cleaning (as well as your heat sink), apply a new coat of thermal paste, check the pins and make sure it's correctly seated.