pull all the spark plug leads off and just rest them on the plugs.make sure you keep them on the correct plug!!!!
then with the engine running, take one lead off at a time, use something that is insulated so you don't get a 'lecky shock!
if the plug and lead are ok the engine will die or splutter. if the plug or lead aren't working there will be no difference in the engine tone/revs.
when you know whick plug/lead it is swop the plug with one of the others (ie. plug 2 suspect. swop with plug 1 ) then retry. if the fault moves with the plug .. it's the plug. if it stays in the same place it's the lead,distributor cap, (clean inside and out & recheck ) or it's not a sparking issue and may get very expensive depending on what it is.
let us know on plug test first. don't forget new plugs can fail very quickly so don't say it's not them 'till you've checked.
good luck.