"Hello everyone i drive a 1993 Isuzu Trooper LS,and just recently i washed my truck's engine and noticed the?

"Hello everyone i drive a 1993 Isuzu Trooper LS,and just recently i washed my truck's engine and noticed the engine has started misfiring on the low speed and seem to have loss of power.It is very sluggish when i press the gas pedal.I really have to go hard on the gas pedal for it move now.I haven't had any problems with this truck.I have replaced the spark plugs and wire set few months ago.So i am wondering if water might have got in to some parts or something.How do i check the spark plugs if they are all firing?Please help me.Thanks."