A cheap, $20 [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXPT31&P=RF]Tower tach[/link] is accurate enough.

All you need to know is when to rule out a reading as false.

You will not see 13,000, when the engine is doing 11,000...
You may, however, see 30,000, with the engine idling, as I've seen happen, when you take readings within close proximity to high-tension wires...

In general, its readings are consistent and quite accurate.