Hi,
You should be able to offset the input signal to get it into the range of 0 to +10 or whatever you want.
For a very very simple example, connect two equal resistors between +10v and ground, say 2k each. That puts +5v at the center of the two resistors. Now using a third higher value resistor like 10k, connect that to the center of the two resistors and connect the other end of the 10k resistor to the input signal. Now when the input signal goes to +10v you get a voltage at the junction of all three resistors of something like 6v, and when the input goes to -10v you get a voltage at the junction of something like 4v. All you have to do then is detect either something like 4.5v or 5.5v to figure out if the signal is high or low.
If the signal varies more then we'd want to increase the junction range. Decreasing the value of the 10k resistor would give us a higher range. We could probably get it up to 0 to +10 for an input of -10 to +10 volts, but this is just a quick example.