You will know the smell of mold when you stick your nose in the bale - lol
I can't describe it but if you can sniff something that's been in your refrigerator a long time, and know it's moldy, you will recognize the obtrusive odor of mold in hay.
If it's really bad, the hay will often have a gray or black shade the mold spores literally pop up at you when you break a bale open.
How old is "older hay" ?? The older hay is, the less nutrients it has. I have fed two year old hay without issue and I've heard others say three year old hay is good; provided the hay were stored properly and didn't get wet.
If the bales have dust on them from sitting, take them outside and give them a vigorous sweeping (on all sides) with a broom.
I would smell every single flake of hay for mold before feeding. If hay was baled damp, it is not uncommon for only part of the bale to mold.
You can also Google "how to tell if hay is moldy" and get some credible hits.
The bottom line is don't feed questionable hay just to save money as it may be a vet bill in the end.
Hope this helps