In the books at least healing is painful, and men’s weaves more so. Men are rarely considered good at healing in general because of this. The energy for healing is drawn from the person themselves, so that becomes a factor as well. SD is correct that healing is tried many times on Rand’s wound by different healers. So, they obviously think it can be done. As far as how they know if they are any good, they try it out under supervision. Even the girls when traveling had the chance to have other channellers around to help or teach most of the early books. For poorly skilled healers, the attempt probably just won’t work. The weaves would touch the wound and do little or nothing. Or as in Liandrin’s case, her healing is said to be particularly painful as well as only having minor skills. Also, there would likely be a prior affinity for it. As with Nyn. Rand does get a teacher, and he will suddenly do weaves he has no knowledge of.

This is a thing. You do have to sign up in each case. But there is .com .co.uk at the very least.