Nokia Mobile Test Modes:

Nokia 12xx, 51xx (except 5125), 61xx, 62xx, 71xx, 3285, 63xx, etc. (Older TDMA/CDMA phones – does not work on newer GSM phones)
Press *3001#12345#. Scroll up 1 to FIELD TEST (some phones may say NET MONITOR instead of FIELD TEST) and SELECT. Scroll to ENABLED and press OK. Turn phone off and then back on. Signal strength is the upper or lower left number. After test mode is enabled it will stay in your regular menu. To easily get in or out, press MENU, up 1 FIELD TEST and SELECT or OK. Dial 00 to get out and 01 to get back in.

Nokia 2100
Press *3001#12345#, MENU, then follow the above instructions.

Nokia 3285, 3360, 6360, (except 3590)
Go through above process. After test mode is enable it will stay in your regular menu. To easily get in or out, press MENU, up 1 to FIELD TEST and SELECT. Dial 2100 to get out and 2101 to get back in (or possible 0000 to get out and 2100 to get in).

Nokia 6215i, 6315
Press ##2773 then press END. Enter 000000 (six zeroes) for the User Lock Code. Signal strength follows RX Level or RX Power. Press END to exit.

Nokia 6255, 6256 and newer CDMA Nokia phones
Follow the regular Nokia instructions above (*3001#12345#, etc). When you power the phone back on, select NET MONITOR from the menu and enter 3202 in the entryfield and press OK to get to the test mode screen. It will show 32-02 in the upper corner of the display. Signal strength is the left negative number. Go back to NET MONITOR and enter 0000 to get out. (Sprint Nokia phones: ##33284#.)

Nokia 6305i
Press Menu (OK), 0. Service Code 000000. Press OK. Choose DEBUG SCREEN. Press OK. Signal Strength is after RX= or RSSI=. This screen also shows EVDO RSSI. To exit, power phone off.
Palm Treo 600, 650, 700p, 700w(x)
Sprint: Press ##33284 and DIAL. The signal strength (RSSI Value) is on the first line. To exit, press CANCEL.
Verizon: Press #*#33284 AND dial. The signal strength (RSSI Value) is on the first line. To exit, press CANCEL.
Cingular: Press #*744625. The signal strength (RSSI Value) is on the first line. To exit, press CANCEL.