Determing the frequency of a signal with a sample at a time
A single sample tells almost nothing about the frequency of a signal.  The only hypothesis that is eliminated is a DC signal of a different value from that of the sample.  

The more samples one accumulates over a longer period of time, the better one might be able to fit a maximum likelihood estimate of higher degree models of what the spectrum of the signal might be to those samples, assuming non-aliased samples of a high enough density relative to the bandwidth the signal.