You didn't mention what data..
You can test for a voltage, it will not be a reliable test as there may be bad data on the line.
For DMX
About the only way I know, You will have to use a spare receiver set to an active channel. Like an Express, MR16, Ather if testing dmx data.
For pixelnet
Pixelnet and smart string can be a little harder to test. This requires some extra thought as to what you are looking for, and how to test for it. This usually involves swapping out parts.
The simplest way I know, keep things in order, and remember how things are hooked up.
I know!! Not much help..