An option is to use a Spectrum Analyzer, using Zero Span mode, video trigger, RBW = 1MHz, etc.
About your question, if is possible to use a power meter, the answer is yes, if the power meter allow you to trigger the GSM burst. Actually this method is more accurate than using a Spectrum Analyzer.
It is another empirical method to use a Power Meter without trigger:
-Set your Power meter on Average mode
-On your mobile use a standard GSM transmission (one TX burst)
-Duty cycle in this case is 1/8 = 0.125
-If you transmit full power in GSM900 (33dBm – PCL5), the Power Meter will read the average power: Pavg = 33*0.125 = 4.1dBm
-To find the real output power of your mobile (Peak Power), Ppk = Pavg*DutyCycle
-These are numbers for conducted power. For radiated power using the internal antenna, you have to subtract antenna gain and path loss. Make a reference and compare with it the other measurements, and in this way you will know if the internal antenna is properly connected.