You get a set of number drills and you try and fit one in the hole, if it fits get the next larger one, and so on until one doesn't fit.
The hole is the size of the last drill that fit. If you don't have a drill size chart, mic the drill.
If you are paranoid about dulling up your drills using the cutting ends, use the shank.
(One drill will always be a perfect fit, that was the drill size used to make the hole in the first place.)

