Unless she responds, or her husband responds, there will be no way to tell she received it. You could try calling her husband directly to see if she got it, or calling her directly.
I'm not a big believer in the effectiveness of the NC letter for actually stopping contact, but I think it does have some value. NC letter won't stop contact. To me its value is in gauging the cheater's willingness to send it. A lot of cheaters push back against sending it. Asking them to send it is one more way to gauge whether they are willing to what is necessary to help you get over it.
If cheaters want to continue contact, if they really want to hide it, there are too many ways for them to do it for you to catch it. They can do it on a burner phone, on phone apps that can be deleted and re-installed without a trace, from work, using a friends' computer or phone - cheaters are usually pretty good at figuring out how to do this, they usually are one step ahead of the betrayed spouses trying to police them. That has been my observation, anyway.
Trust your gut and pay attention to how he acts. You were in the dark before, now you probably can recognize some signs, some changes in his normal personality and behavior when he was in the affair. If you see that again, you can start your sleuthing.