Generally speaking, the first one to change states is the leading edge, the last the trailing edge, but it really depends on what signal is being looked for. If the circuit expects to see a negative going voltage as the valid trigger then the leading edge will be the first one to drop below zero volts and the trailing edge will be the first one that goes back towards zero. Usually there will be a minimum expected voltage, such as once it passes -.3v it counts as a valid signal, this is to minimize noise's effect on the circuit causing false triggers.

I'm using a lot of caveats here simply because I don't know the actual circuit design and am guessing on a lot of it. If I had the schematic I could, eventually, sort out how it's supposed to work.