For instance check whether the distance between your player and the object is small enough:
dx = object position x(certainObject) - playerX
dy = object position y(certainObject) - playerY
dz = object position z(certainObject) - playerZ
dis = sqrt(dx*dx + dy*dy + dz*dz)
if dis < minimumDistance
moveOnToNextLevel()
endif
Or just using
object collision(playerObject, certainObject) instead of the distance.
How you realize the level progression depends on your code and how you load/create/store levels in general. Can't really tell you anything about that with the information you gave us.
