Frequently, there will be plugs removed from the insulation to allow the Inspection group access to the pipe to take ultrasonic readings to monitor corrosion. Talk to an insulation supply company for some suppliers of these.
If you are something like foamglass rather than fiberglass, this might not be practical.
Try NDT Sales of Houston, Tel 800-261-6261. We bought plugs to penetrate fiberglass insulation and measure steam line wall thickness. Plugs come with caps that close the opening and can be pulled out for re-measurements.
Use an ice pick! I do a lot of "field walk downs" and have to determine pipe sizes. A lot of the time I'm dealing with insulated steam & flow lines. So I found using an ice pick lets me accurately determine insulation thickness, from there I can also determine pipe size. All with minimum damage to aluminum cladding. Good luck! ...Mark