How do you know when a clam is dead?

Okay, like a week ago, my little sister went to a park near the ocean and found a live closed small clam, as big as a regular small cookie, and it was white, and well my mom said lets keep it in a little tank until we go back supposedly the next day but we didnt go, and well it just layed there on the tank closed, on top of big rocks, and well today thurdays, an hour ago after i me and my family came from jack in the box, the clam opened, widely and my mom like 'its sleepin with his mouth opened ' -_- & well, idgaf but then right now im home alone and i softly poked its toungue or whatever that thing inside is, and nothing happened i then taught it was dead and tried to closed the clam, and it didnt, so wtf happened, should i just trow it out or go to the ocean and trow it out? Or if its alive what should i do with it?