Where I work, (shipping an recieving type company), we have pallets coming an going, believe what Ed said. They have all sorts of things spilled on them. We've even encountered many that have been painted, mainly used for food shipping but even those pallets end up being used at other places.
The ones that I look for are: color of the wood, (visible stains, etc.), how many times they've been repaired. As stated above, you don't want to hit an unseen nail or screw. Check for an odd odor, if you smell other than wood, stay clear of it!
I personally look for ones that are fairly new. You work with pallets long enough you can tell the difference. Another note, as Ed stated, pallets tend to sit outside in the weather... I've seen birds attempt to build nests in them within a few days of sitting.
Just to say, there are pro's an con's using pallets for a cheap wood source. Since you mentioned possibly using them for practice cuts, this wouldn't be a problem, provided that you wear atleast a dust respirator. I'd wouldn't use them for anything except an out-door type project... bird house, planter, that type of thing.
Also, as Mike mentioned, use a metal detector to check, double check an recheck again for any unseen metal.
Ken
