Hi there, although it is not a weir it may have some similar characteristics to one. The problem with large concrete structures is their uniform construction, that is, a large flat face that water streams down. This uniform face can create a wide, uniform and retentive hole that will hold a boat or swimmer for hours.
The best way to test it is just to see what the shape of the hole is and if there are any breaks in it. If the edges of the hole face downstream, it is relatively safe and you could work your way to the edge to get out, if the hole faces upstream then it is going to be retentive and much harder to get out of. Also if there are a 'gates' or gaps in the structure then the waves/holes formed downstream of it will be broken too and more likely to contain spouts that will flush you out if you get stuck. The only other thing with man-made structures is their form beneath the water, sometimes they have hidden features that can snag and hold you, like metal bars, holes etc.
If you're worried about how it will perform and can't predict it from the shape of the wave pile and hole you could try sending something that will float into it and see if it can find it's own way out. If a barrel, log or empty boat can then a swimmer should be able to as well and visa versa!
Good luck