Here is a link to a short article I wrote on that subject.
Shortr answer is these often become clogged with debrie and the moisture hopefully will soak through the morter.
You could get real detailed and use a boroscope.
If new you should see those little wicks that deteriorate over time so warn the client never to tuckpoint over them.
As much as weep holes are needed every 24" at the base of a brick veneer, it is just as important to provide and weep above doors and windows where the same counter flashing material has supposedly been installed.
Weep holes are require per the IRC 703.7.6.
I have provided you a like for open weep holes that you may encounter also.
Hope this helps.
Great link with easy to understand pics.
Almost as good as mine. lol