How to determine if the ooth fertile

How can I tell if my praying mantis ooth is fertile? It’s a big question and even the most experienced people are rarely sure. 
Determining whether an ooth is fertile within 100% certainties is difficult. Short of it hatching out nymphs of course being definitive. 
There are some ways that I use which can help. Infertility can be decided by some obvious factors.

1. Lone female of species not known for parthenogenesis (self reproduction)
2. Ootheca is malformed, size, color, shape and form are irregular. It takes some experience and knowledge of the species of ooth your examining and not always clearly seen.
3. The amount of time for that species of ooth to incubate has long expired. 
4. Visually some species have translucent qualities and embryo development may be seen by a trained eye.
5. By examining the ooth surgically or microscopically. Opening an ooth isn’t recommended, there will be some loos of embryos, and if done improperly can damage and kill all of it.

This is a fertile ooth. Even though it was cut open only a few of the cells with embryos were destroyed, the rest of the ooth can still hatch and so with white paper glue on the foam area and the perimeter it can be resealed

Breaking open the ooth is an extreme test, however if it is something you want to try in order to salvage it if prematurely done and you find it to be fertile, you can slice a clean straight cut with razor to the end open approximately 1/2” or 1cm to evaluate whether it is fertile. If there are cells with living embryos in it then carefully put it back in place and glue it best as possible with white paper glue (Elmer’s) or equivalent carefully glue it around the perimeter avoiding the ‘seam’ or hatch line where the embryos would

Shown above here closing ooth once examined with paste.

See below resealing neatly for viability