I believe the best option for pest free chaeto is a seller that specializes in selling macroalgae and specifically advertises that they sell pest free chaeto.
They won't keep it with other corals either. Kinda like their own isolated systems. If it's in a sump system that corals are added to in lfs always a chance of getting aptasias in it.
These things can be crazy tiny to the point it looks clean until it hits your system. A few weeks they grow larger and start spreading because they often. Have lots more nutrients to take in verse the often. Filled coral tanks as lfs compared to our home systems.
Another safe option is to keep the cheato in tiny tanks by itself heater some flow and a little light, maybe sprinkle a little bit of reef roids or similar every few days dobserved it for 2-3 weeks and see if you start seeing any. Within a month you will either know it's clean or has things in it.
The ok it looks clean and tossing it in ur sump, is nothing short of doing that experiment in your tank display tank.