I have no idea what kind of bars re on that bike since you didn't mention it. But it's easy enough to figure out the diameter, understanding that downturn bars are almost always 31.8 mm these days, and if not, 26.0. It might even say on the bars under the stem clamp face plate, or on the stem itself. Flat bars are sometimes 31.8, sometimes 25.4.

It's simple to use and very cheap to buy a simple Vernier Caliper at any hardware or big box store for $10-20 which will be accurate enough to tell the difference between these normal sizes. In case it matters for cost, you don't need digital accuracy for these sort of measurements.

Or bring it into any bike store or anyone you know who has a decent set of tools, they should be able to measure it for you.