Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
Output Transformer Shunt Method
With the amp turned off, attach the black lead of your current meter to the plate of the power tube being measured (on most octal tubes this is pin #3). Then attach the red lead to the center tap of the output transformer (this wire usually connects to a large filter capacitor and/or the standby switch, but you'll need verify this on the schematic to be sure).
NOTE - When you power up, your meter will have hundreds of volts going through it. So use alligator clips rather than holding probes on the test points manually. (You'll save yourself from potentially damaging your amp or yourself if you accidentally slip and ground out the plate voltage).
Now when you power up, if your amp has 2 power tubes, you'll be reading the current through 1 of the tubes. If your amp has 4 power tubes, you'll actually be reading the sum of the current being drawn by 2 tubes. As long as you're using matched tubes, you'll just divide this value by 2 to get the actual bias current.
Finally, adjust the bias pot so that the current reading is the appropriate value for the type of tube your amp uses, (see the list below). Then let the amp warm up and note if the bias changes significantly. If so, select a bias point that is between the two readings.
And you're done !
Currents Per Tube - (Class AB1 Operation)
6V6 --------------- 22 to 27 ma
EL-84/6BQ5 ------ 22 to 27 ma
6L6/5881/KT66 -- 30 to 35 ma
EL-34/6CA7 ------ 35 to 40 ma. (sometimes higher)
6550 -------------- 40 to 50 ma
If money is less important than ease, then you can pick from a wide range of easy to use biasing tools that are available in an equally wide range of prices. Here are a few.