The most reliable way to diagnose this type of problem would be by measuring the actual current that flows through your treadmill. That's likely what an electrician would end up doing once all the normal trouble-shooting doesn't lead anywhere. It should never sustain more than 15A, although a brief spike would likely be fine.

You can pay a couple of hours worth of time for an electrician, or you can buy a digital clamp meter. I linked to the cheapest model that I could find; it's only about $20. I am not saying this is high-quality device, but the price is right.

Clamp meters are highly accurate and would tell you exactly how much current flows through your tread mill. All you have to do is set it to measuring amps and then close the clamp around one of the wires going to your treadmill. You can either do this by splitting a two-conductor extension cord, or you buy a relatively inexpensive line splitter for $16. Amazon has cheaper models, but they looked a little unsafe, so maybe go for the good brand name.

Alternatively, a Kill-A-Watt works similarly, but lacks some of the more useful diagnostic features that you get with a clamp meter. It really depends on whether you think this is a one-off or whether you expect to have other electrical issues that you think you'll need to track every so often.