Can I measure the -3dB point of a wireless link with a lower frequency?
Assuming your transmitter is NTSC, you have 4.2MHz to play with, according to Wikipedia.

To answer the original question, yes, it is possible to measure a signal at a lower frequency (using aliasing), but you must be able to sample at atleast twice the bandwidth of interest, and the signal band must start at a multiple of your sampling rate.