if it is a brand like HP look on back it will have sticker w/ serial/model

if it is custom... turn it off, unplug it, open case.... whatever devices aren't working... look for largest chip or printing on boards: ie (intel, asus, 3com,etc)... you need to pull make then model... the model will be printed on boards look for largest chip or board...

see example: below is an abit_fatality_f-i90hd