How to diagnose battery problem

[INS-ANI]

Hey, i own a inspiron 1420. For last one month i have been facing some battery issues. One day it will be online, the other day it will be offline.
I am unable to diagnose if my battery is dead.
Can anybody advice me if anything can be done about it before i report to service center as my last resort.

Also, the laptop specs are:

• Screen: 14.1-inch WXGA+ (1440 x 900) UltraSharp TFT Display with TrueLife
• Processor: 1.66GHz Intel Core 2 Duo T5450 ,intel 965 chiset
• Hard Drive: 160 GB hard drive (5400RPM)
• Memory: 2GB RAM (PC5300, 667 MHz, DDR2 SDRAM)

it was bought almost 2.5 years ago and performance is as usually down. Is it advisable/possible to upgrade its specs? I have little idea if i can upgrade CPU or Add GPU in a lappy or with the current CPU upgrading RAM will have any considerable effect on performance.
Please advice on it as well.