How to test two-prong retrofit outlets downstream of GFCI
Testing the initial GFCI receptacle should be sufficient. No need to test the load side receptacles since they are protected by the GFCI. If the GFCI works, and the 2 prong receptacles are connected to the load side of the GFCI, then they are protected.

If however, you wish to pursue it, you would need a separate conductor run to a good ground. Insert the tester into a 3 prong to 2 prong adapter and attach the ground to the ground prong tab (or wire) of the adapter. Then plug in to each of the old 2 prong receptacles to test.

Good luck!