i had a similar issue with mine, my mechanic suggested that it was the injectors and i should run some injector cleaner through them. still resolving the issue myself though.

you should be able to just visually check the plugs, there are pics of what plugs should look like (and what they shouldnt look like) in most workshop manuals...i havent actually checked the plugs on my wrx yet, but my brother did changed them on his without much hassle afaik

but a good diagnostic process would include injectors, plugs, leads, coil pack... AFM
i'm sure the mech guru's on here will have a better idea than me, but just make sure you do it one thing at a time, so you know what the problem was ie: dont change the plugs and the leads at the same time etc