FYI, multiple APs are supposed to share the same SSID, provided they are connected to the same physical network. What you don't want is them on overlapping channels near each other. Remember, there are only 3 channels that don't overlap, (1, 6, 11), so you should do your best to keep the channels from conflicting. They will work on overlapping channels, but will have to wait for each other to talk, essentially defeating the purpose of having multiple APs to begin with.
If you are looking for basic tools, simply running Kismet with a decent card that can have an antenna attached is helpful.
Otherwise, consider getting a spectrum analyzer. They are great for tracking down sources of interference, and visualizing what is actually going on.