Aweosme, so just a basic RODI.
Here is what I would do, as I always want to know my membrane efficiency.
I would move the "tee" with the "out" sensor to the line that runs into the the set of DI cartridges and get a reading from there.
Easiest way to do this will to just use a bucket and move the sensor.
Start by pushing the push lock in for the sensor and remove it from both pieces of rodi line.
Then find the line that runs from the ro membrane to the DI cartridges.
Remove that line from the DI cartridge and place the "tee" on the end of it.
Set that in the bucket and start making some water. After 5 minutes or so, note the number for the TDS from the meter. Then put everything back to how it was.
This will allow you to see the membrane effciency.
Also, may be a good idea to go ahead and replace that sediment filter. If the filter is really bad looking once you get it out, you may just want to replace the carbon filters as well, since there really inst much of test ofr them, unless you have some chlorine testing strips (like for a pool)
If you do end up replacing the carbon, they will need to be rinsed and we can into that if needed.