The one key thing needed for all roles, not only tanks. "Communication." We can't expect to know if the DPS and healer are good enough to take on multiple mobs. Not every healer can handle them. Not every DPS is capable of producing enough AOE DPS to kill large mobs fast enough. It all comes down to conversation. The most basic form of tool taught to us since early in our lives. As a main tank myself, I always ask when I join a pug if they are good with large pulls. You never know unless you ask.