Note down the value of sampling rate of your data acquisition system. Call it "Rs" for simplicity. Sampling rate of a data acquisition system is defined as the number of times it can acquire a sample of an input signal per second.
Divide the sampling rate by two to calculate the Niquist frquency for your system. For example, if the sampling rate of your system is 10 Ms/s (10,000,000 samples per second), the Niquist frequency of your system will be 5MHz. Call it "Ns" for simplicity.
Note the frequency of the signal that has to be sampled using your data acquisition system. Call it "Fs" for simplicity. Calculate the closest integer multiple of the sampling rate "Rs" to the frequency of the sampled signal. Call it "Rint" for simplicity. For example, if the samling rate is 10Ms/s and the frequency of the sampled signal is 56 MHz, the closest integer multiple would be 5.
Calculate the alias frequency (Falias) for your system using the formula: "Falias = Absolute((Rs*Rint) - Fs)."