Well, let's start off by spotting before you can troll him. First check his (or her) house. Is it dirt? If not, he (or she) could still be a noob. Check what tool he is using and on what. here are some noob combos:

Pick section:

Pick = Wood
Pick = Dirt
Pick = Mobs

Axe section:

Axe = Cobble (complete noob!)
Axe = Dirt

Shovel section:

Shovel = Wood
Shovel = Cobble (No, cobble is not gravel!)
Shovel = Mobs (It is NOT a spear!!!)

Hoe section:

Hoe = Cobble (this isn't half of a pick....)
Hoe = Wood
Hoe = Mobs (not a scythe...)

Also, noobs ask a lot like "What is that green hissing thing?" yep, that's a nooby question. Soo a noob.

You got a noob, troll him!

Lava or TNT are your best options.