Ace check means: A quick verification that someone or something is performing at their best and hasn't slipped; a spot-check for excellence. Also used as a call-and-response to confirm that everything is going. It has drug-related associations; worth a calm conversation about which side of the topic your teen is on. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.