First, I'll give credit where credit is due, Instructables has a great, easy article on this. Click here to go there.
Size;
The general size of a pallet is 48 X 40. If the person you're getting pallets from hasn't confirmed the size for you, take a tape measure. When I went to get pallets most of them were 42 X 42. If you have a project in mind when you're getting the pallets, consider the different sizes. Of course if your project involves dismantling the pallet, it doesn't matter so much.
Symbols;
Symbols are the main thing that are going to tell you about the pallet.
IPPC. If you don't see the IPPC logo, be very wary. While the pallet may still be safe, it could have been treated with chemicals.
HT - Heat Treated. Heat treated pallets are not harmful, and heat treatment is a preferable type of pallet to get for that reason.
MB - Methyl Bromide Fumigation. DO NOT USE PALLETS STAMPED MB. Some countries have banned Methyl Bromide treatment because of the health risks to pallet workers. These pallets are treated with a harmful chemical.
DB - De Barked. Pallets marked DB, have some from de-barked trees. There is usually no problem with DB pallets.
That's pretty much the major things you should consider before getting your pallets, but always be sure to wash them with bleach and water, or power wash them before you use them inside your house.

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