Using an ammeter as you describe would work fine, but the topics this thread including mine should come with a footnote[1] at least. Especially when this thread shows a simple mistake can cause a lot of confusion about what to do. My thought process with my comment about capacity was that muscle memory makes it likely you'll eventually start the car if you're touching the ignition switch/button during troubleshooting, although on further thought and reading the other comments I'd do the following: pull the starter fuse, then use a 10A ammeter function of a common multimeter, connected inline between the negative harness and the negative terminal and not on the positive side.[1]

[1]This is just what I would do. Only do this based on a reputable guide if you don't have experience working around car batteries.