An application can guide the system in determining which textures should remain resident by specifying a priority for each texture using glPrioritizeTexturesEXT(). Each priority value is clamped to the range 0 to 1, with 0 indicating the lowest priority and hence the least likelihood of being resident, and 1 indicating the highest priority. You can also set the priority of a single texture by calling glTexParameter*() with GL_TEXTURE_PRIORITY_EXT. Note that the performance effects, if any, of setting texture priorities are entirely system dependent.
You can query whether a set of texture objects is resident by calling glAreTexturesResidentEXT(), and you can query whether a single texture object is resident by calling glGetTexParameter*() with GL_TEXTURE_RESIDENT_EXT.