I know you can pull the plugs & if one in particular is more fouled than the others it could be a sign of a leaky injector. Or with the engine running try spraying some carb cleaner around the base of the injectors. If the o-rings are going bad the engine will sputter because the carb cleaner is being sucked into the combustion chamber. Im not sure if any of this could be the cause of your problem. Theyre just a few tests ive learned along the way.

Good Luck, Matt