In tomato, go to Device List. This will show you every machine's MAC and IP that are connected to the network. Identify all the machines on the network that should be there and eliminate the one that you don't recognize.
This is not quite what the OP was asking, however. He was having an IP conflict, which tells me that his computer was set with a static IP address. While his computer was disconnected, another machine on the network, which was set to allow the router to assign its IP address automatically, preempted him and assigned the IP address that he wanted reserved for his own computer to that other machine. His problem could simply have been alleviated by manually assigning his computer a different static IP address at the time of conflict, setting his computer to allow the router to assign a dynamic IP address, or assigning all the machines on his network static IP addresses that did not conflict. That's probably why no one addressed his problem at the time. It wasn't a problem at the router, but on his machines.
Hope this helps,