Logic probes are a cost effective diagnostics tool for testing electronic circuits. They allow you to quickly see if a pin is held high or low and see if the state of that pin is changing where expected. Although not a replacement for on oscilloscope, a logic probe can help you troubleshoot a lot of basic issues.
The Elenco Electronics LP-560 Logic Probe provides convenient and precise measurement of Logic circuits by displayign logic levels (high or low), pulses and voltage transients down to 25 nanoseconds. This model will allow you to test all logic families (ttl, lsi, CMOS, ect)