is there anyway of telling what i've done to the phone if they plug it in at the store?
If they are so inclined, sometimes they can figure it out, depending on how bad it is bricked and how knowledgable they are.
However most of the time, if you are paying the money, they just try to reboot into recovery a few times and give up if it doesn't work. Often if you are paying money, they just verify it doesn't boot, and order your refurb.
Incidentally, I think you have a one year warranty from LG, but Sprint won't handle those anymore (outside of 30 days), unless you pay $35 to "expedite" it, meaning order you a refurb. Your refurb then only has a 30-day warranty even if your original phone had 11 months of warranty left.
If you wanted to avoid the $35 fee and send it to LG for warranty repair, LG would probably fix it for free under warranty, but when you got the phone back, Sprint's written policy is to apply the $10 smartphone/premium data fee to your line when you reactivate the smartphone.