Is there a way to detect what speed modem a oerson is using..?

Try Pink Pack Pro at http://www.ipswitch.com. It has a throughput tab where if you know the ip address of hostname of anything on the web, you can get an estimate of the throughtput from your machine to theirs. Granted, it doesn't give you their modem speed exactly, but you can estimate it based on the results.

In terms of web development:
If you talking about using a cgi or asp script to determine the speed on the fly so you can give the visitor a hi-res or low-res site then you can't, not without much delay and programming. Have the visitor select their modem speed from a pull-down box.