You can buy a hay probe that measures moisture and temperature of a bale. They are expensive (about $200 last time I looked) but reassuring. I believe at 150 degrees you pull the hay out of the barn, once it hits about 180 degrees it cooks off any moisture, flashes up to over 300 degrees then you get spontaneous combustion. The farmers will tell you if it starts to smell like caramel it's to hot. You can use the moisture % when your hay shopping much over 20 odd % when newly baled (moisture content spikes for a day or two after baling, then drops as the hay cures) and it is likely to mold, once cured it should be under 18%. Round bales are harder to combust because you can't pack them as tight in the loft.
I also salt my hay as I stack it, my grandfather always did and it can't hurt.