You can remove it and hook the battery to the coil contacts and detect if they close with an ohm meter or you can turn on the ingition and remove an injector plug and check if you have 12 volts on one of the plug pins. Some people have said you need to remove all the plugs to check voltage but I didn't find that to be the case with my car. The other pin should have zero. Backfireing is usually caused by too rich a gas mixture or bad timing. Your fuel pressure regulator may be leaking gas through the vacuum hose or maybe the camm belt has jumped a tooth.