Have a look at this cool PIC 12F683 Microcontroller Based Logic Probe.

“The purpose of this device is to test a wire to determine the logic state. Two LEDs show the logic state as either high, low, or high impedance (aka floating). Simply attach the ground probe to any ground connection of the circuit you want to test, then touch the test probe to the signal you want to test.

First, this device determine if the wire is floating, this is done by applying a weak current into it with a pin on the microcontroller and see if the state changes, and draw a weak current from it using the pin and see if it changes. The current is weakened by the 10k resistor attached to the pin, commonly called a pull up or pull down resistor if used for this purpose (overriding the state of a floating wire). If the state changes, it means the wire is floating, and the device indicate that using a LED.

If it’s not floating, then the voltage on the wire should indicate a logic high or low, and the device indicate that using another LED.”