If you mean identifying a plant as some kind of mint, it is easy. The mints all have sturdy square stems, opposite leaves that are crinkly, and a menthol aroma when crushed.
If you mean identifying peppermint specifically, to distinguish between peppermint and spearmint and other mints, it gets a lot more tricky.
the flowers of the spearmint plant grow only on a terminal cluster, which is tapered. The blossoms of peppermint grow on a terminal, which is whorled rather than tapered. The leaves of Peppermint can be described as sharply toothed while those of spearmint are oblong, and lanced
https://www.helium.com/items/1189776-spe ... d-contrast
You can see that they look nearly identical, but if you look closely the teeth on the peppermint leaves are sharp, while on the spearmint leaf they are rounded. The flowers would probably be more distinguishable.
This site has a slide show of different mints: