I'd be surprised if they've only seen a week of rain being scaffold boards. They are the unloved and much abused of woods
I'm sure others will have better ways of telling but, without a moisture meter, one way would be to weigh them every few weeks. As the moisture dries out they will become lighter. When the difference in each weigh-in stops they're pretty dry.
They will split, shake, cup and all sorts. That's a certainty I'm afraid. They'll also likely be full of dirt, stones, staples, nails and other things that will ding blades in a blink. A metal detector (£15 security guard wand type) comes in handy for finding the worst of it. I certainly wouldn't be putting any good tools near it.
Also, eye protection is a must (minimum) for the sake of bits or metal or stone going ping at very high speed assuming power tools are to be used.
if you can bring them indoors inside a building that'll dry them out fast, ideally leave them a few months in stick indoors and you can't go wrong. Make sure you seal the ends though with paint, I like chalk paint, helps minimise cracks n splits.