This is it, right here ^. This is the easiest indicator; depending on the amount of cover in the tank, it is easily possible that they are hidden somewhere out of sight. The behaviour of the parents is a dead giveaway. If they are acting aggressively protective towards you, then just relax and keep your eyes open for that wonderful little cloud of fry in a couple days. It's one of the little joys of fishkeeping.
Worst case scenario? They ate 'em; very common with young parents on their first spawning. Not the end of the world, they'll lay again soon and practice makes perfect. Take a breath, relax, and let nature take its course.