Well, that's also overly simple.

Let's say you have a pretty good idea of your actual MHR. Not by some formula, but from your own data over a significant period of time.

If you one day see a HR that is well above your supposed MHR, the most likely reason is that your HRM is doing something anomalous. But if you have good reason to suspect that the reading is real, then you may be experiencing atrial fibrillation and indeed, you should get it checked out.

If on some other day you are doing a hard effort where you expect to see a HR near your MHR, and you can't get your HR up to any where near the expected value, then you may simply be under-hydrated, over-fatigued, or have some kind of unrecognized illness (something coming on, something that is otherwise mostly asymptomatic, getting over the flu, etc.), I wouldn't run to the doc just for that. Get some rest and see what happens on another day.
If your MHR, well tested under good conditions, suddenly has dropped a bunch (more than 10 bpm, or so) and this is repeatable over a period of time, then maybe think about getting it checked out.