I was in the same boat as you for a while. I have 4 RIR hens and at the start they all looked alike. In time, their combs grew out differently - some flopped over, some were bigger. It helped me to tell them apart. I also came to know over time who like to lay in which nest. Unfortunately 3/4 like the same one! Typical chicken law!
You could try some coloured leg bands to identify them. Usually, mine will lay in their own favourite nest, so it would give you enough information to be able to say "Red band lays in the window nest" and "Green band lays in the corner nest."
They are quite tricky though, and can change nests for no apparent reason if the mood takes them!
I have also heard of people putting different coloured food dye around the chickens vent, so you can see if Red Bird or Green bird laid that day. I have no idea if this is safe to do or not, and don't do it myself, but I have heard of it being done :/
Hope this give you some ideas.
- Krista