It's often a judgement call and when you replace the board/cpu/part, if it works, you were right.
For the Dell, there are some with indicator LEDS on the back. Did you use those for some "code" to figure it out?
Bought this computer in October 03. Last week it just stopped working. Called Dell; they stated it was either the motherboard or the processor. Took it to the repair shop today and they told me (without looking at it) that it must be the motherboard and it cost a lost of money. They will look at it later. I have a little knowledge and would like to replace the motherboard myself. Can anyone offer any help in doing this if in fact it is the motherboard?