Changing RO/DI filters?

I'm thinking its time to order new filters for my RO/DI unit but I've never done this before. I have a bulk reef supply four stage RO DI unit. I bought this used a long time ago and never thought to change filters then and I'm sure I'm way overdue now. My main question is, how do I know which filters to replace? How do I know if the DI resin needs to be replaced? 
My clue to needing something replaced is now when my 'clean' water overflows my blue collection jugs, it leaves a white film on my tile after drying. I don't have a TDS meter but thats got to be quite a bit of crud in the water if it leaves an obvious mark on my floor. 
Thanks,
Kylie

 

Never mind everyone. Bulkreefsupply is awesome and I found a great video teaching me. For anyone else who didn't know much about their RO/DI unit and needs a little boost of knowledge, here's the link:
http://www.bulkreefsupply.com/store/products/reverse-osmosis-filters-and-systems/replacement-filters-and-di-resin

 

Thats what I'm getting /DesktopModules/ActiveForums/themes/_default//emoticons/smile.gif I ordered a dual line TDS meter to instal on my RO unit so I can catch when I need new filters before its obvious on my floor /DesktopModules/ActiveForums/themes/_default//emoticons/tongue.gif