You should be able to find the answer in any literature that may have been provided (usually on one of the supplied CDs), you could look up the system specs for your model on the Dell website, or even email/call their tech support people.
If you happen to have a USB 2.0 device handy, you can always plug it in and see what happens as well.
A third option is checking in the Device Manager for an "EHCI" listing, or "Enhanced Host Controller Interface". If you see that, you have USB 2.0 ports.
How can I tell if I have USB 1 or 2 ports?
Belarc advisor doesn't tell me much.
Dell 4700, Win XP home SP2.