multiple lights on one switch

I already have multiple lights on one switch with power entering the light fixture first. I have 3 wire cable coming from the main panel and have been able to look at how all wires are connected in the run so it will be a simple task to replicate this setup with the new lights I am installing elsewhere. My problem is I'm not too sure how the wire running from the last light to the switch is connected to that switch. It might have been simple to see those connections at the switch but there has been a smoke detector also connected at the switch and I can't tell which wires are coming from the last light from the wire used for the smoke detector. So, my question is when I run my 3 wire cable from the last light to the switch, where do I connect the black, red and neutral, if in fact, they all do get connected to the switch.