The car suddenly started to run rough.

How can you tell the difference between an ignition coil failure and an ignition control module failure on a GM 3800 engine? The dealer diagnosed a failed coil (there are three on this vehicle) but wanted too much money to fix it. I replaced the coil myself, but that didn't solve the problem. I brought the vehicle to another shop who, knowing that I already replaced the coil, recommended replacing the ignition control module. That took care of the problem. I'd like to know how I might have got the right diagnosis first.

Thanks.