How To Tell If You Need A New Filter For Air Purifier?

Hey folks,
I was given a HoneyWell 52500 air purifier a while back from a friend. I'm not sure how much it was used or if the filter or pre-filter was replaced because his parents gave it to him.
Is there anyway I can tell if it needs a new prefilter and/or a new filter? Together they cost nearly $100, so I'd rather not just buy it for the heck of it.
Thanks!