There are different methods for tuning. For example, you can use a network analyzer or vary the frequency.
Using a network analyzer is the preferred method because there are less sources of error, and the network analyzer gives actual results. For a given measurement, the network analyzer shows the resonance frequency for the tag antenna. See Network analyzer for more information.
The method varying the frequency uses an oscilloscope to measure the maximum voltage swing. This method has more sources of errors and gives relative results. The maximum voltage swing among the results is what is of interest, one measurement only will not give much value. See Varying the frequency for more information.
For the final results, it will not matter which method was used as long as the Near Field Communication (NFC) tag antenna resonates at 13.56 MHz after tuning.