Just look at the hardware and make sure it seems sturdy. After only 6 years, things shouldn't be bending, twisting, etc. Make sure that there aren't TOO many gouges in the shells- it's a used kit, but it's not 20+ years old, either. The big concern to me always involves toms- be sure that the adjustment and attachment system for the toms are functional, strong, and that YOU can live with them. I've seen some hair-brained systems in my time...Since I am still a newbie when it comes to all this gear, what are things I should look for? Other than the obvious cracks/damage, is there anything that I should pay attention to in terms of the cymbals or components? Should I bring a tuning key and play with the adjustments? Aside form the components not being rusty or damaged, making sure they move freely, what else can I look at with the hardware and kit?
Check the cymbals all around the edge and the center hole. Tiny cracks now can become big cracks later... especially if you're young and not very nice to your cymbals. 6 year old cymbals will likely be dirty, but they should still be relatively shiny- they are A Customs, after all.
If the drums don't sound good right now, don't stress over that. Most of it can be fixed with heads. Do, however, check bass drum hoops (especially if they're wood)- be sure that they're not cracked and are in round. If the snare sounds terrible, check the strainer mechanisms to be sure that they function properly and it is just a tuning issue. I once brought home a traded piccolo snare only to discover that the throw-off couldn't stay engaged when any tension was put on the strainer. I had to tie it in place, which is frustrating.
Check out the pedals- hi hat and bass. Make sure that they run smoothly and don't rub anywhere along their motion. Sometimes springs get deformed and start causing problems, although this shouldn't be an issue after only 6 years. Make sure that any adjustment mechanisms look clean and functional- your taste will likely change over the coming years, and you'll want to be able to adjust the pedals to always be as comfortable as possible.
If you have pictures, you should post here so we can help you further.
One more thing. If there are any serious problems, or even if you don't like it as much as you thought, don't be afraid to walk away. Your perfect kit will come; if it's not today, it might be tomorrow or next week or next month. It's better to wait and buy something that you're totally happy with than to pull the trigger on something that you don't totally love.
That said, big problems CAN be cause to negotiate the price down. Point out the issues if necessary, and negotiate appropriately. Take a guess at what it might cost to repair some things, and deduct that from the total cost. You can also tell the seller that you will buy the kit at the agreed price... IF he will arrange for the necessary repairs.
Best of luck to you. Happy hunting!