Hi, that's not irrelevant at all as I often wonder that as well. There isn't really a way to tell if a wild ootheca is fertile or not. Since mantises lay ooths regardless if they mate or not it could be non viable. And determining whether they have hatched yet or not can also be hard but it is doable.
So how I determine if an ooth is hatched is too look at the seam of the ooth, for instance the top part on a Carolina ooth, and see if you can see any tan fluffy specks, which is the nymph breaking open the foamy part, and also if you can see a tiny hole that looks like a capsule has come out, which is where the nymph emerged. Now if the ooth is old you won't see specks easily.
Or if the part pointing downward looks like it has opened up. Though sometimes this doesn't work.
Another thing to look for is to see how weathered the ooth looks. Like does it have holes in it? Lichen or mold growing on it? Does it look very dark and wrinkled? If it looks very old and damaged most likely it is an old ooth that won't hatch.
And you can always keep an eye on the ooth to see if in the spring there are any baby mantises around the area. Native mantises usually hatch around March to May.
I hope this helps! Have a blessed day!
