I have tried measuring bores w/ inexpensive telescoping bore gages. They are 2 finger, spring-loaded "T" shaped with a knob to lock the dimension, pull out and measure w/ a caliper. I have trouble getting consistent readings, usually varying ~3 mils. They seem to work best when you lightly tighten the knob, then tilt them back & forth so the cyl diameter pushes them in. May be good enough to help decide if you need a rebore = buy new pistons, maybe scrap engine when you see the price.
I get the best readings by placing a new piston ring in the bore, pushing down mid-way and square w/ a piston, and measuring the end gap. If in spec (usually <12 mils?), can just re-hone. Of course, the problem is you must buy new rings first, thus start w/ method 1. Of course, if your old ring gaps are in spec, you are already there, assuming standard ones.
You can also measure w/ feeler gages between the bore and piston skirt to determine if piston slap would be a problem. There is actually a way to knurl the skirt metal so it fits tighter, but don't know if commonly done. There are special piston rings that overlap so no end gap, or buy oversized ones and file to size.
Many will recommend just take to a shop and pay big bucks. Reasons to re-ring yourself:
1) Shops are happy to rebuild a fine engine. When you overbore, you can't go back
2) Rings have much less surface area than the cylinder wall, so most wear occurs to the rings. If you can still see factory honing marks everywhere in the cylinder, it can't be very worn (unless re-honed already). Honing scratches are ~1 mil deep?
3) If you didn't hear piston slap before, you probably won't after a re-hone and re-ring.
4) It isn't hard to hone and re-ring with the block in the car. No need to unbolt from tranny, lines, remove hood (try getting that back right and not scratching paint).
5) If pulling the heads anyway, and engine >100K miles since the last re-ring, smart to replace the rings. New rings will only make it better. De-glazing cylinders always helps oil control, sealing and wear.
If an in-car re-ring, always remove the rear main bearing and change the rear seal. If the bearing looks good (usually), I don't change main bearings. Might also use plastigage. You can replace them with the crank in. Always replace rod bearings since you remove the connecting rods, and they usually look worse ("ant farm" tracks from carbon).
On my 2.4L Plymouth, I had to pulled the head at 97K to replace the badly designed head gasket w/ a new MLS type. I took the head in for a valve job (probably unneeded) and re-ringed in car. The new ring gaps were well within spec and the cylinders only slightly glazed. It was probably unneeded, but not much extra time and parts. Now I know the engine should be fine for another 200k miles. Note: on newer engines (Magnums up), you can't replace the rear seal without removing the transmission, but they are a better seal.
