There is no universal way to decipher that.
If your question is about the Hardware, it ought to have been presented differently.
You could boot into the BIOS and check the specs of the rig if it supports 64Bit computing, that ought to answer the question.
Or else post the specs (ie. Make , model) of the processor and mobo here and we'd tell you if it's 64bit capable of 32bit limited.
Or you could search the same on the internet and get the answer.
Actually the OP can use CPU-Z or HWInfo and it usually will indicate a 64 or 32bit system.
AMD FX(tm)-4100 quad core(3.6GHz),ASUS 5MA78L-M LX PLUS mobo,4GB RAM,500GB HD etc etc
You can use either CPU-Z
to take the guess work out for you.