There's some information provided by Apple regarding batteries:
Which one of MacBooks are you using? The last generation (unibody) or discrete one?
For the MacBook (unibody), it is said that your battery should retain approximately 80% of its charge after about 1000 full charge cycles. For the discrete MacBook, it's about 300 cycles.
You can check the battery charge cycle by going to the System Profile as another member has suggested. Or you can download an application called 'Coconut Battery'. It provides you with detailed information about your battery.
Here's the battery information:
Serial Number: 9G0120DNVE1MA
Device Name: bq20z451
Pack Lot Code: 0
PCB Lot Code: 0
Firmware Version: 201
Hardware Revision: 2
Cell Revision: 158
Charge Remaining (mAh): 3623
Fully Charged: No
Full Charge Capacity (mAh): 5396
Cycle Count: 331
Battery Installed: Yes
Amperage (mA): 691
And how about the problem of the macbook gets hot? I never put the macbook anywhere near fire, the environment is absolutely cool and I even have the fan on...