Reverse Engineering: TTL/LVTTL signal direction
As "helloworld922" said, put a low value in-line resistor (you can probably get away with something as high as 100R, but I'd go for something like 10R), and also pull both sides of that resistor to GND with equal, high-value resistors (e.g. 10k).  You should be able to see the voltage drop on a "high" signal.