The best thing is to find a seller who you trust who will tell you the truth about where the corals have been and who displays in his/her shop corals/invert tanks that are kept separated from fish tanks and water circulated through fish tanks. Also, soft corals that are not attached to rocks are safe, and all inverts that do not have hard shell surfaces are safe. So many corals are safe. Live rock, live sand coming from tanks, inverts with shells, sps, and soft corals that come attached to rocks pose some risk.
Personally, I would not QT corals for more than 3 weeks, and that is all I QT fish as well. Ich will show up on fish within 3 weeks if it is there.