MB21 - Any guitar, mail bought or otherwise, should be thoroughly checked as I'm sure you are already prepared to do! Obviously the first thing you'll check is the asthetics of the instrument. Checking for any signs of cracks or dings etc. Along the lines of Spud's suggestion I would also remind you to make sure that the truss rod has enough "adjustment" - either way - tight or loose...so that you have the ability to straighten out the neck when it needs it...or enough threaded adjustment so that you can loosen the rod for changes in humidity, altitude or neck relief. The neck won't be any good if you notice you can't loosen the truss rod for string relief and it's already as loose as it can get (free spinning). A good neck should have enough thread to go either way for a good adjustment. Also make sure you can intonate the thing. Some bridges are good some aren't. Make sure you can play chords up along the neck and in the open nut positions once it's intonated without it being off anywhere at all.
I'd also check how the guitar was packaged too. Inspect the box even before you open it...that should tell you a lot about how it was handled in delivery.
Guitars/Bass - MIM Fender Classic 50s Strat, MIM Fender Standard Strat, Squier Classic Vibe 50s Tele, Gibson Les Paul Studio, Epi '56 Gold Top Les Paul, Martin DSR acoustic, Sigma Martin Auditorium electric/acoustic, Squier Jazz Bass.
Amps/Cabinets/Modelers - Model 2558 50 watt Marshall Silver Anniversary Jubilee combo w/ Celestion Vintage 30s, 4x12 Marshall cabinet w/25 watt Greenback Celestions, Fender Blues Junior w/ a couple of Billm mods, Line 6 POD 2.0, Roland Micro Cube
Pedals/Effects - Cry Baby Classic Wah, Boss TU-2, Boss NS-2, Boss RC-2 Loop Station, Ross Compressor, MXR Micro Amp, Danelectro FAB Echo, Danelectro FAB Chorus, Danelectro Chicken Salad, Marshall Guv'nor Plus, Marshall Echohead, Duhvoodooman's Zonkin' Yellow Screamer, Digitech Digiverb, Digitech Bad Monkey, Dunlop Fuzz Face, Homemade Loop Bypass pedal, Duhvoodooman's Sonic Tonic (Maxon SD-9 clone +), Voodoo Labs Superfuzz