There isn't that I know of, unless you have a micrometer. If you had some strings that you already know the gauge of, you should be able to compare the flexibility and stiffness and figure out which is a match.

What is already on the guitar is most likely 9's or 10's. If you are totally in doubt, start with 10's.

On flat- vs. round-wound, in theory the flat wound cause less fret wear, and create less handling noise (hearing your fingers move etc), but round wounds have better tone. I used to see flat wound commonly back in the day, but round seems more popular in this decade (based on what I see on the store shelves). I used to use flat, but use round wound now.