You will need another CPU as a sanity check, its highly probably current one received 12V. better yet another working slot1 system to check all components (ram, vga).
I assume you did plug PC speaker?
Post card is your friend, they are $4 Free International Shipping nowadays.
You can use keyboard in lieu of a post card as a makeshift indicator for running CPU:
-connect only motherboard/ram/cpu/supply and keyboard
-power on, watch for keyboard leds blinking - keyboard controller initialization is very early in the boot sequence
also try with no ram and listen for pcspeaker, beeping for no ram is even earlier in BIOS boot sequence.
Alternatively you can make your very own post card with 74LS138, 74LS374 or 74LS688, 74LS244 and 8 leds or $4 CY7C68013A ebay board.
..or just stick oscilloscope/logic probe on isa address/data bus (or BIOS chip legs) and watch for any activity.
You might find this thread helpful to get an idea of how to approach debugging:
https://www.eevblog.com/forum/repair/how-to-troubleshoot-your-defective-386486-motherboard-with-an-oscilloscope/ Its possible CPU was killed by 12V and while dying passed it to the chipset. Are you up for BGA replacement lessons? might be fun!