Thanks. What type of 'reference source' do you have in mind?
It can be some oscillator with stable and known amplitude. For example you can use some TCXO powered from REF voltage source and ask someone who own calibrated RF voltmeter to measure it, then you can use it as reference source.
Another but less reliable and less precise way is to measure some low frequency oscillator (about 10 MHz) with 200-300 MHz bandwidth oscilloscope, it should give you result within about 5-10% error. Then use it to calibrate logarithmic detector module from such 10 MHz reference and then use logarithmic detector module to measure 100 MHz signal.
The logarithmic detector allows you to use low frequency (about 5-10 MHz) source for calibration and then measure much higher frequency.
If you have oscilloscope with >= 400 MHz bandwidth, then you can measure 100 MHz signal amplitude directly with oscilloscope. But oscilloscope measurement is not so precise, the error will be about 5-10% or even worse if frequency is more than 1/3 oscilloscope bandwidth.
Another option is to use
tinySA Ultra, it can measure signal level, show signal spectrum and can be used as reference source, but its cost about 100 USD.
If you decide to use tinySA Ultra or logarithmic-detector, then also buy
20 dB attenuator and connect it through attenuator, because 2.34Vpp (which is +11.4 dBm at 50Ω) is pretty high level. With attenuator you will need to add attenuator value to measured signal power in dBm.
So, it depends on your budget.