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Try looking at the end grain of the wood. Cupping aside, if you do not have vertical grain wood, the core side (center of the tree side) should be down. You can rip cuts on the underside (strong 1/8th inch;3/16th inch) lengthwise an inch or so on center to help stabilize the plank. Also, if you have the time and space, you can sticker the wood on a dead level surface and sticker heavier planks on top and let gravity work for a little while. The further away from a center grain and the closer to vertical, the more stable the wood. A common spec/request I make is "free of core". I would take a blemished plank with a tight knot vertical grain over a knot free center core plank any day of the week.

Also, when attaching pre drill (prevent splitting) really screw/bolt the outer ends down. The sun is cruel on long wide planks.

Garris