First three stages are typical going to be a mixture of carbon and sediment filters, and often you'll have 2 of one kind and 1 of the other, a very common setup is a sediment filter then 2 carbon blocks, 2 just as a back up to make sure all chloramines are pulled out. The problem is carbon really doesn't have any color indicators to let you know that it's ready to change, any color that's on them is due to sediment that gets through that filter and really isn't what those filters are removing. Having some sort of flow meter that records how much water goes through them is the most ideal situation, however an easy second step is to have a tee with a valve right after the first 3 filters and periodically test the output with chlorine test strips, and the easiest way is to see how fast your DI resin gets used up, if you notice a very rapid change then it's pulling out more stuff than it was likely chloramines.
