You're going off-road so you want winter tyres or at least all season tyres, with a fair amount of profile still on it. Tyres exposed to sunlight get bad much faster, so look for a pile of tyres that isn't exposed to sunlight, or ignore the ones on top of the pile that are.
Look for tyres that have their thread pattern evenly worn, ie. don't take one that has much thread on the middle but not on the side. This tyre has been insufficiently inflated a lot. If there's only thread on the sides that means the driver has driven really fast or the tyre has been inflated too much for a long time. As @Cc Dd said, look for damage on the tyre. The date stamp doesn't necessarily say much, because it can be that the tyre is taken in use a year later than that.
When you find a tyre you like, try to deform it a bit to see if it's still in fair condition. Judge it on its flexibility, does it bend and deform easily or not. Car tyres are rather stiff because of the steel weave in it, but there's always difference in flexibility between one and another tyre.
Try to bend the side of the tyre as much as possible and check if any cracks shop up. Cracks often become visible only this way. The tyre size requirement is obvious to you, but also take speed rating in account, although you probably won't be driving that fast off-road.
Lastly, there are better and worse tyre brands and brand series. In case of off-road driving, you have to ignore every single tyre that has words like 'Eco contact' or 'Fuel save' or anything on it.