You may not need to adjust the tension, but you may nevertheless want to measure the belt tension, at least to ensure it's not too tight: the shaft on stepper motor has limitations.
In that case, no need to buy a tool, you can use an audio app showing the frequency spectrum and then you pluck the belt. The highest frequency peak can be related to the tension as explained here:
For GT2 belts and 27 N tension, the relation is
f [Hz] = 28531 / length [mm]