(40Hz's right)

The hack you can do is to additionally sample at, say, 97Hz as well as 100Hz. A 97Hz sampling rate will then see a 60 Hz aliased to 40Hz signal go from 40Hz to 37Hz, showing that your signal is probably actually really 60Hz and not 40Hz. If it was 40Hz at 100 and 97Hz sampling frequency, then it's probably actually 40Hz.

(It's been a looong time since signals class though.)