Measuring Peak Output Power for Frequency Hopping Transmitter (915MHz) for FCC Section 15.247
Antenna port conducted measurement is usually the preferred method for measuring power. If the measurement is not possible due to device having PCB or SMD antenna, radiated measurement is also allowed. If your device have antenna port, you can use connect coax cable to attenuator then to spectrum analyser.

You might want to see ANSI C63.10 for the measurement method. There is also a KDB related to 15.247 measurement that might be helpful for you.

Mind that if your device employ less than 50 hopping channels the limit is 24 dBm instead of 30 dBm. 

If you need more reference, go to FCC website and look for test report that do 15.247. They usually have pictures of test setup.