Can someone suggest a simple, if there is one, method to determine cylinder throat diameter?

I'll guess you are talking about a Revolver.

1. Pin Gauges will give you the Narrowest Diameter.
2. Slightly Compress a Lead Bullet so it Expands just enough to be Tight in the Cylinder. Then Lube it well and Drive it through from the Case Head End. Then measure for the Widest and Narrowest Diameter with 0.0001" capable Micrometers to see if they are slightly Ovate. You need to do this for each Cylinder.

Best of luck to you.

HotCore is right, but pin gages are soooo much easier.