Query number of mipmap levels

It generates the mipmaps from the first after the base level down to the 1x1-texture, or the level defined by the GL_TEXTURE_MAX_LEVEL parameter. Thus, the number of additional mipmap levels it generates is n=min(floor(log[sub]2[/sub](size)), L[sub]max[/sub])-1, where size is the maximum dimension of the texture, and L[sub]max[/sub] is the value of the GL_TEXTURE_MAX_LEVEL parameter.

Since number of mipmaps that was generated depends on a parameter other than the texture ID, you cannot uniquely determine how many mipmaps it really generated from the texture ID alone. If you assume that the max level has not changed, you can query the size of the texture and calculate the number of mipmaps that was generated as n=floor(log[sub]2[/sub](size))-1.

so OpenGL has no function for query the number of mipmap levels? How can i know then if a texture mip pyramid was truncated by the user?

No, there is no way to query the number of mimpaps in a texture as far as I remember. But since there is no way to remove individual mipmaps then there is no way to truncate the chain of mipmaps either. The only way to not have a full set of mipmaps down to the 1x1 texture though, is to limit the mipmap levels when creating the texture in the first place.