Carbide Tips for Toolholders
I have a couple of carbide tip holders without the tips; in both cases for the rhomboid shaped tips. Is it possible to measure up the holders and determine what designation tip is needed (CNMG 120408 etc?)
Yes, but it might be easier to determine the correct insert by checking with the manufacturer of the holder based on the part number stamped/etched on the shank.
There are some subtle differences between inserts and their holders that might escape you in measuring. Holders for CCMT & CPMT inserts would be an example of a slight difference in holders that'd be hard to detect by measuring.