You can judge by the appearance of the manure. If you have horses you know what fresh horse manure looks like. Brown with rollers and mixed with bedding if you use it. As it's actively composting it will look kind of grey. But if you look closely it's a mix of white and black pieces, don't ask me why. While it's grey it will be steaming. After that period it will be black.
I'd say that you could garden with the grey or the black; however you'd be safer, seed wise, to use the black.
You're still going to get weed seeds because they'll have blown onto the pile. And the weeds will grow very robustly, but so will your crops.
On level ground a manure pile will be the oldest closer to the barn. on a hillside each side of the barn exits to the ground at a different level, so on one side there will be a manure pile. if that's the case the oldest will be at the bottom. I would guess this possibility would be the easiest to judge. On the level option, someone may have dug into it near the barn and it was recently refilled.
The site I'm working has multiple piles on a steep hillside. As one filled up and got dangerous they moved. I've been working that pile for two years and there's the remains of a third pile. I've used both old piles and they grow wonderful crops. I'd suggest you scrape away the top surface to get rid of any blown seeds. Even though my pile is that old I do get a few weed seeds, but they're those big grain plants; which I don't mind as they grow tall and are easy to pull without bending over; at my age.