SDL_VideoDriverName man page
SDL_VideoDriverName — Obtain the name of the video driver
char *SDL_VideoDriverName(char *namebuf, int maxlen);
The buffer pointed to by namebuf is filled up to a maximum of maxlen characters (include the NULL terminator) with the name of the initialised video driver. The driver name is a simple one word identifier like "x11" or "windib".
Returns NULL if video has not been initialised with SDL_Init or a pointer to namebuf otherwise.