what would it be called?

Something you stick down the barrel and tells you how many in how far .

would a gun smith tell you that?

you can just do it with a rod and jag.

https://www.youtube.com/watch?v=3YmMNpbFjp0
thanks

Some rifles nowadays have the twist rate stamped on the barrel, so you don't have to guess or experiment.

And on the ones that don't, often there will be factory specs available (either online, in the manual, or by calling the company) that specify the twist rate used.