You can check your Thai Baht balance for your phone using a USSD Code. Enter this number in your phone and dial:
DTAC Happy press:*101# or *101*9#
AIS 12Call press: *121#
True Move press: #123#
Here are more free USSD codes.
Checking your balance
You can check your balance for free using the above USSD codes. If you are roaming you can also check your balance while you are roaming. If you have just started a data plan you might want to check your balance a few times to be sure you are not being charged per Kilobite for your data usage.