Port 1 of network analyzer to one end of diff circuit. Port 2 of network analyzer to other end of diff circuit. Take the S-Parameters. Put this S-parameter block in your favorite simulator. If it is @/D/S, then terminate the block with a 50 ohm termination with both ends ungrounded. Then you can read the differential S-parameters return loss. If your simulator only has grounded terminations (M/W/O), then use an ideal 1:1 transformer between the S-parameter block to the termination in order to do the diff to single ended conversion.
If you don't have a simulator, you can also calculate the diff return loss using the single ended S parameters taken above.
See R/F D/e/s/i/g/n M/a/g/a/z/i/n/e Jan 1999 page 24.