One test is to measure the resistance across the primary terminals, yours should be 3 ohms. This doesn't tell you much because a coil can measure fine but fail under a load.
The best way is to buy two spark testers at the auto parts store. From one coil you connect one spark tester to each of the secondary wires and ground the other end to the engine. You can crank the engine over and see the spark. Set the gap wide enough to load the coil, a good coil will easily jump a 3/4" or 1" gap. Also look at the color of the spark, a blue spark has more energy than a yellow or orange colored spark.