How do I tell if the tires on my mtb were mounted tubeless onto the wheels? The combo is tubeless compatible, I just want to make sure they were actually installed tubeless.

Long story short, two of us purchased mtb's from the same shop within the same month. Both were supposed to be installed tubeless, he got a flat the first weekend on the bike and come to find out they didn't install his tubeless. Giving them the benefit of the doubt of a miscommunication for now.