Your best bet is to get a moisture meter, like this one:
Amazon link to one meter. Moisture meters come various ranges: you want one that measures in the 6-14% range or so. Anything above 14% is not useful for woodworking as it will shrink too much.
Meters come in two varieties, pinned and pinless. The pinned meters measure stick two electrodes into the wood about 1/4" and measure the electrical resistance. You tell the meter what species you're measuring, and it uses an internal table to tell you the moisture content.
The pinless variety won't mar your wood, but they're quite a bit more expensive. They use induction or ESP or fairy dust or something to read the moisture level.
If your wallet is glued shut, you can use weight to measure the moisture content. Have a look here.
You don't need to fill in the cracks to prevent further cracking. You need to seal the wood to prevent further cracking/checking. Moisture is wood's undoing, and you want to keep the level of moisture in the wood consistent year-round.
Almost all wood sealants are aimed at doing this--paint, stain, poly, etc. For outdoor furniture, poly-based stains are REALLY good, if you don't mind the slightly rubbery feeling they can leave. For stairs, look into decking products, some of which are poly-based, but are formulated to allow you to walk safely on them.
Now, if you have big cracks, you might get snow and ice in there, and the ice could wedge the wood further apart. Depending on how big the cracks are, you can fill them with epoxy resin, and then seal the whole thing with paint/stain/etc. The stain won't sink into the epoxy and color it, so you'll need to mix some colorant into the resin as you apply it.
If the cracks are even larger, you might consider using something like a Dutchman (bowtie-shaped piece of wood) to hold the two pieces from splitting further apart. This takes some skill and either a router or chisel and a good eye.
If the cracks are enormous, you may just be better off replacing the wood where it has split. Make it match as best you can, and then make sure to put a good coat of sealant on both old and new wood.
Best Answer
From my experience with Walnut, etc.
You slab it with the bark on, oversize so there is excess stock to work with when planed to final dimension and stack the flitches with stickers between them to keep them separated. The base they're stacked on needs to be flat, the storage sheltered from rain and extremes of weather.
Paint the end grain to slow down moisture movement in aid of reducing checking and minimizing splitting.
When properly dried out, the flitches get resawn and then sent to the planer for finished dimension or sent to the veneer mill to be sliced down into sheets.
A local business, Wagner Electronics, produces moisture meters that are used to determine when wood is seasoned to the proper moisture content. Slow is good. Note: not an affiliate of any kind with the company, just relating how the professionals determine seasoning speed and finish.
Flitch: a piece of wood that is to be resawn or sliced for veneer.
Sticker: thin pieces of wood used to separate flitches or veneer stacks.