If your content is coming from a verified supplier, then they'll supply you with individual certificates for each model.
You can't verify it any other way really. If you're just copying & pasting from sites, then you can't guarantee it, but you can assume that if they're displaying their "All models were over 18 at the time that the photos were taken" license, then they all were.
But still, copyrights are there to prevent you from doing this. Unless you paid for the royalties to use the models, then you're breaking the law.