Is the purpose of coring to determine non-conforming material or to determine why something has happened. You need to locate the rebar prior to coring. I know of no qualitative manner of determining coring location, generally visual and based on experience.
With respect to the first item results of coring tests are often inconclusive, subjective and contentious. A clear understanding of the objective is required and any remediation. This should be established prior to coring, with all parties in agreement. My personal experience with coring for this item has been dismal. Coring may not be the panacea.
With respect to the second item, coring is always an interesting endeavour. You need to core where the damage has occurred, but also in an area of reduced stress to prevent damage to the structure. It is often that the area examined is a region of high stress. This where experience is required.
Dik