How to tell current frequency band?
Based on avirk's hint, I found a way. First press the code *#0011#

GSM example:

GSM900: IDLE
T: 10, B: 10
Rx Pwr: -94, Rx ual: -
Rx Lev: 21, Tx Lev: 0-
Speech VER: AMR EFR FR
VOC: AdaptiveRate
TS: 0
Temp: 68 Batt: 71 LNA: 0
Service: Available


It is obvious that it's using 900MHz band, from the "GSM900". 

UMTS WCDMA example:

WCDMA: Idle
Rx CH: 10612, R: -94
Tx CH: 9662, Tx Pwr: -
EdIo: -6, RSCP: -98
SpeechVER: AMR EFR FR
RF: Sleep2
L1: PCH_Sleep
Drx cycle: 64, PSC: 392
Temp: 67 Batt: 69 LNA: 0
Service: Available


UMTS can use two different physical layers (low level radio) UMTS-FDD and UMTS-TDD. Android just shows WCDMA, which I think is the same as UMTS-FDD, but perhaps it can refer to both of them. By looking up the channel number here you can tell the frequency bands and whether it is UMTS-FDD or UMTS-TDD. 

In the above example it is using UMTS-FDD, the down-link channel (Rx) is in the 2100MHz band and the up-link channel (Tx) is in the 1900MHz band.