It is not so much "Who claimed the dependent" as "Who is entitled to claim the dependent"

This tool will help with that determination.

https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent

Lot's of times people claim dependent's they are not entitled to, which does not stop the proper person from claiming the dependent, it only stops them from efiling, if the other person claimed them first.