Stupid question. Doesn't the router keep logs? If it does, then all you have to do is go through the logs and you'll know if you're getting scanned. If it can, but logging is turned off, turn it on. If it can't you shouldn't be using such a crap piece of hardware anyway, dump it and get a 386 running Slackware.
Cheers,
cgkanchi

Edit: I meant that my question was stupid ("Doesn't the router keep logs"), not yours. On rereading the post it seemed to me that it might have been misinterpreted.
Cheers,
cgkanchi