iStat menus can show you the KB amount coming in and going out but not the signal strength.
The software that came with your modem should show the signal or if you have a phone on the same network check the signal with that
If your 3G USB modem is similar to the ones I use the signal strength means very little... the number of people using the bandwidth is far more important.
5 bar signal & no bandwidth available to use is far too common here. School's out @ 3.30pm. forget it...