How can I be certain the logic board is good/bad

Hello,

Our iMac lost its’ HDD shortly after warranty had expired.

We have never had Mac that didn’t croak so I always have a fresh boot disk that is created every other day. We have recently (about a year ago) had trouble where the external wouldn’t even boot. Booting into boot manager the only options were for an internet recovery.

I was able to try all boot options and finally the external boot drive would after having unplugged everything and gradually plugging everything back in.

Fast forward ahead a few months and the only way to restart is via boot manager (option key) but it was reliable. Fast forward ahead a bit more and not only did my external boot drive appear but also the internal HDD reappeared! I was even able to boot from it and back up the contents before it disappeared again.

We are now in a state where boot manage works SOMETIMES but only if I unplug everything, start boot manager then add the external drive while boot manager. Oh and the internal HDD is back but the computer freezes I try to boot with it. Sorry for the belabored end of this question.

Basically, it’s like the whole data bus just drops out of sight. I have tried USB, Thunderbolt connections and when having issues the computer sees no drives at all. I want to assume it is the logic board but how can I tell ultimately?

Thank you so much.