Scale samples. They can actually be very fascinating...for a good introduction to the use of scale samples and various age compositions of steelhead check out The Steelhead Trout, Steelhead Flyfishing, both by Trey Combs and Steelhead Water by Bob Arnold.

In general...the rings on the scale will be very close together or even show an area where the scale shrunk as steelies dont 'feed' in the freshwater and thus do not grow.