I wish we all had CMM's! Shouldn't this be a human right?
I usually do the "reverse engineering" with a vernier caliper, measuring the hole diameters and distances from adjacent edges to various features.
If there are a lot of holes, then typically a lot of them would be in some kind of regular spaced pattern. If possible, I try to reason out what the part is being used for, whether it is a component from a "metric-scaled" machine or "inch-scaled". This can help a lot to determine what a measurement
should be.
A quick overview of the part to gain this kind of perspective is liable to pay off, rather than slavishly copying what may not have been laid out all that accurately to start with.