Depending on the level of knowledge of the user you are concerned about, it may be possible to view details of past net use by looking at index.dat files.
These are not cleared by the normal processes of deleting IE History and Temp files, details here;
http://www.acesoft.net/delete_index.dat_files.htm If you scroll to the foot of the page, you will see a link to download a free index.dat viewer. If you install it, (it's called @Winspy), it will list details of various logs kept by Windows. Note that some security programs may flag @Winspy as malware since it is collecting information on your system like certain worms/trojans do.
You should be aware that it is possible to land on 'dubious' sites by accident and that even if your current suspicions appear to be confirmed by reading these logs, you should think carefully before confronting anyone with 'evidence' you have obtained by, in effect, 'spying' on them.
I am a parent myself and if I were in your shoes, I would consider options like filtering software before going for the 'nuclear' option of a major row. When I was young, curioisity was catered for by certain types of magazine you didn't want your mother to see!