How will i tell if my pet turtle is dead or not?

i just got them yesterday and i moved them into a bigger tank. i put little rocks down and some water so there are dry and wet patches in there. they were moving when i put them in there and i checked on them last nigth and they were both tucked into their shells and one buried itself under a few little pebbles under some water? the other is right near it and they havent moved since last night....dead or alive?