How do you know what model number an Inspiron is?

I was given an Inspiron that was bought in 2003 but there is no model number on it and the previous owner does not know.  I have the invoice but that just says Inspiron SPUT?  Which isn't a model.  Looking at the images it looks like a 9400 but how can I be sure.  I want to know because it has problems and I want to disassemble it to look at the fans but want to know the model first,

 

thanks,

Clodagh.