Actually, FM transmitters' output power can easily be measured by a power meter within its frequency range and terminated with a resistive load of 50 ohms. Meter commonly used are Bird Thruline by broadcast engineers. Antenna and other peripherals has nothing to do with the transmitter output if your purpose is just to determine the RF power out. Knowing the Field strength after propagation from antenna is not easy to determine TX out, since transmission line loses, antenna gain on its front lobe, distance etc., are needed.
In the absence to VHF power meter, and since you're the one who did assembled the transmitter, you can take a look on the power transistor or tube characteristic curve and have the voltage supply level you used and operating frequency can have an approximate power it can generate. Sometimes, operating efficiency of the power output stage is required to know its factor for the true power. Broadcast transmitters has a standard load impedance of 50 ohms.
And since it's your thesis, I'm willing to help you if you so desired.