Our content mostly has Open Graph information set. I think this is done by Yoast SEO. Do you think this is correct?

There are quite a few (~1200) URLs that have no Open Graph image set.

I am thinking I can write a functions.php action to check to see if the Open Graph image has been set, and if not, supply a fallback OG image.

How would I check to see if an OG image has not been set?

Help appreciated.