If so, how?

A society without genomic testing would have to rely on the visible fact of twinhood — on the actual presence of twins in a community — to identify them.

Twins are identifiable in any community: they are the children born together, the siblings who look identical, the unusual pair whose existence carries cosmological significance in a culture that understands twinhood as meaningful.

If the ritual practice at this chultún specifically sought twins, twin children would have been known and identifiable in the communities from which sacrificial individuals were drawn.