Your thumb is your best tool. Yes, really. Take one lentil out of the pot.
Press it against the roof of your mouth. Or, if you’re fancy, press it between your thumb and finger.
Does it smush easily? Does it fall apart with a gentle squeeze? Done. If it resists like a tiny pebble, it needs more time.
Think of it like a handshake. A cooked lentil gives you a soft, confident grip. An undercooked lentil acts like a clam rejecting your fist bump.