Well, easiest way is when you have the problem, look in your tasktray and see if the wireless or network icons shows disconnection.
Or I guess another definitive way is to open two command prompt windows, and one of them, run "ping <router IP> -t" and the other run "ping
www.google.ca -t" and when the problem happens, see if one or both of them stop getting replies.
If your router still replies, then for sure the connection to your router is fine, but the Internet connection is dead. If neither of them replies, then your router is causing the problem.