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If you mean what speeds the network will handle, that can't be done with an app, it has to be done with hardware. (You need an adapter you can force to one mode, then try to connect. Then force to the next mode and try. Etc.)
Sounds to me like he's looking for something to determine whether a wifi network is running B/G/N, much like data networks running 3g/HSPA+/4G/LTE.