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
Your drawing is showing a staggered pattern in the boards. Is this your intent? If so, you'll need something to screw down to at every butt joint. I see one joist at the end but nothing else to hold it up. Next, you won't find many mills who will "cut to length", so every 2x6 deck plank will be 6' or 8' long. It's easier to put down the 8 footers, then snap a chalk line on your edge and trim them all down with a circular saw at one time - a good saw blade is required here.
While on the joists subject, I am NOT a small guy, and 6' between joists seems quite far apart for a 2x6, at least for my size (6'4" 326 lbs.) Go to the lumber store, pull a 2x6 beam and a pair of 4x4s to lay it across, and measure how much it deflects when you stand on it, and then have someone else step on it momentarily.... (Again, I am not the guy to stand on a 2' cantilever edge of a deck, but that's up to you and code enforcement for your location.)
You may want to consider having your lumber run the other direction, and put some joist spans every 4 feet on center, reducing your cutoffs, and simplifying the process. Draw in where you expect the tails of the boards to fall, and get an idea as to what the deck will look like when you're done. If you don't need it to run away from the house, then I'd go with rotating boards by 90 and drop in joists at the appropriate lengths.
The general rule of spacing between boards is a screw/nail thickness - mostly because they're handy. Boards shrink and swell with the weather, so they need a bit of gap in them, plus you don't want water sitting down in those gaps, or you'll have rot in no time. I would use the "actual" wood dimensions and forgo the gap in the math (2x6 == 1-1/2" x 5-1/2" in reality) - it'll only be one to two boards difference at best for a deck this size, and it's not like you will not make a bad cut, find a warped board, etc.
Boards come in 6' and 8' and up. The longer, the more expensive per foot, as it's harder to cut a 16' straight board than an 8' one.
Let's say you want to do north/south, in your drawing. For your short section (using 8' beams cut down to 7'): 182" / 5.625" (I put in 1/8" for gap) = 32.33, aka 33 boards minimum.
Long section 168" / 5.625" = 29.86 or 30 boards, assuming you by 16' ones. If you assume you will use a combo of 6' and 8', you'd need 30 of each. and if you want to go all 8' and cut down, it'd be 60 boards.
If you go east/west, you can stagger by 4 feet, adding some look to the deck, but you will have to add some joists, plus move your "main joists" out to the edges, or build trimmer boxes along the entire edge to hold up the tails. It looks like you are after the simplest method, so to do this one, move the posts.
84" / 5.625" = 15 boards running the length of 350" (just under 30 feet.) If you use 4' on center joists, with 8' planks, you'd use 4 boards per run or 60 boards, for the short deck to run the whole length across. 96" / 5.625" = 17.06 = rounds up to 18 boards, by less than 3/8ths of an inch, so I'd just mask it by gapping just a teeny bit wider on the last few boards. Put in the end board and lay the other 3 or 4 down without mounting, take a tape and calculate out the gaps needed. The 14' feet width is a bit awkward so you'll use both 6' and 8' (18 of each flavor) and you'll have to add at least one extra joist, on one side, to continue the stagger neatly.
So, a lot more wood, but a lot more look too.