Aliased signal has phase shift due to Nyquist
I agree with mkeith's answer, but would add that if you have the liberty of changing the sampling rate, you can distinguish an aliased from a non-aliased signal. 

For example, if you sample at 140 MHz instead of 150 MHz, a 15 MHz signal still shows up at 15 MHz. A 135 MHz signal shows up at 5 MHz instead of at 15 MHz. I have actually used this observation before to figure out if the signal I was looking at was aliased or not.