If you're running the current beta, dial *3001#12345#* and hit Call. You'll get the above screen. Notice how the signal bars in the upper left corner have been replaced by a negative number? The smaller (i.e. less negative) the number is, the lousier your signal is. In other words, a reading of -79 would be a worse signal than the -104 shown here.
The field test returned in an abbreviated form in iOS 4.1, but now it appears to be back in full force.
[Via Cult of Mac]