You're right about these being Benningtons.
As to how modern marbles get into older collections ... it almost always manages to happen. Someone knows where the marble jar is and slips some newer ones in there. Maybe.
And as to how to tell ... oh, that is a long story. Sometimes colors or textures. Often structure. This one doesn't look too very new. Could be from an intermediate period. Vintage is usually considered before 1970. Maybe this is from around then. But the colors and structure look _more_ like a modern Mexican or Asian marble than they do like the closest American marbles they resemble. The way the ribbons join at the short seam in your picture here makes this one look closest to being a Master marble, if it were American. That is a "seam" or a "cutline", by the way, not a pontil in this photo. Pontil would be for a handmade or handgathered marble. This is machinemade.