We should concern ourselves only with the functions provided by the SDK header files that need to be linked agaist gsmbas.lib. There is no generally recommended way to find out which functions a dll includes.
An example of a header that uses gsmbas.dll is etelbgsm.h, which is used to get a device IMEI code, for instance.
However, for your interest, it is possible to find out which dll is needed for a certain class. You can find more information on this: