Hehe, there's a question so silly, even I can answer.
Best way is to ask the previous owner! Otherwise, yeah, bumpstops are a reasonable way - but you don't measure the actual bumpstop! Instead, measure the distance between the bumpstop and the point it will contact. Bumpstops are tall rubber blocks or cylinders, either on the axle, or more often on the chassis rail. Either way, under compression of that wheel, the bumpstop stops the axle crashing into the chassis, or otherwise moving too far.
At ride height, on flat ground, measure the distance I mentioned on both sides, take the average, and compare it with your model's standard measurement.
Hmm, re-reading your question, I think you might have coil springs, and the stop is on the inside of those? In that case, just measure the height of the spring!