Yep, visual evidence of cutting charges is pretty hard to miss. It doesn't cut so much as tear and you get a lot of deformation from the explosive charge that propells the metal slug. So the flange of the girder is deformed below and above the cut, as well as a very ragged tear through the metal, very different to shearing or other modes of failure.
There is a picture of a metal girder from the site that was supposedly cut using cutting charges, but it's far too neat to be HE and the angle is wrong, you have to cut at 90 degrees to the surface otherwise you lose nearly all your energy to the air. I don't think it was Jazzz that found it but it's around on one 911 CT site or another and it annoys me.