Usually, as instruments have single end input, you should convert diff to single end by using a balun, tuned in the correct freq band.
Then power can be injected into an instrument, such a spectrum analyzer. Of course you have to add balun losses to your measurement.
Take care of the impedance seen by your circuit. For example, being input impedance of RF instruments 50 Ohm, using a 4:1 balun, your device will be loaded with 200 Ohm differential, so you will measure the power coming from your device when it is loaded with 200 Ohm.
I hope it can help.
Mazz