I use pin gauges for small holes, these can be had in set with .001"
steps. I got mine offf the scratch and dent table at MSC, they
regularly run sales on them. This will not tell you how oval the hole
is, only that the hole will pass a certain pin size.

Usually the engine overhaul manual will have specs as to how much side
play is allowable on a valve stem, mesured either with a dial indicator
or a factory jig. In used heads, the guides are usually worn tapered,
oval or both. Guide replacement is usually fairly easy, I'd still get
a machine shop to do it if this is your first head and you are actually
planning on using the head in something you want tor run for a long
time. Experience counts, here.

Stan