I would pop it open next time you do a water change just to see what it looks like. You won't be able to tell a big difference in the flow unless it drops alot. So pop it open and just look if nothing else. It never hurts to look.
You may want to at least hook up a hose to the bottom drainage port and expel whatever debris has accumulated at the bottom of the filter. You can do this with the filter running.
Wow! I clean mine once a month.I thought I would have to,with the 30 plus african cichlids I have in my 125gal.I noticed that I still have nice water flow as well before cleaning.