I'm DMing for one day in the game I play and I've made the enemies. One is a TRex. I wrote it all down but I'm not so sure about one thing in the beastiary. They can swallow someone whole only if they're grappled. It has the ability "grab" but what kind of check do I or the team have to make to see if they're grappled and if they stay grappled? What check do we make, who makes it and what do we have to roll to see?