Not certain but I doubt that a typical multimeter would register the voltages found. You might need to connect a resistor across the aerial outputs and measure the drop across it. Let us know how you get on.
A very strong output signal will be of the order of 0.1Vac, many cheap multimeters can measure to this voltage – but not at a frequency of over 300MHz (which is where the frequencies of interest will lie).
Your best bet is to buy a spectrum analyser - which will display signal strength against frequency. Look to spend at least £10,000 on one to meet your needs.