Of the pic with the 3 filters, the one on the rt is the DI. Your sediment filter on left looks like it could be replaced. Should be white when new, looks to have particles, (brown) all over it.
Do you have a pressure gauge? If so you could hook it up after the carbon filter and then to before the first, sediment filter and compare readings. If you have at least 60 psi at both places, the filters are still good.
The path of the water in your system should be:
sediment, carbon, RO membrane, DI. Most systems have two carbon filters to ensure the RO membrane does not receive any chlorine, which deteriorates the RO membrane.
The following in no way promotes this co. I have never bought from them, but the diagram on page 8 might help you. In your case, the 5th stage would be the DI
http://www.watergeneral.com/support/pdf/RO585.pdf