Urine and blood tests can differentiate between methadone and other opioids in the system. It's not designed to be a cure (although methadone doses can be slowly reduced in order to break the addiction completely, this has to be done with some care to prevent them going back to heroin), it's designed to stabilise them and give them a clean, legal, free source of drugs which satisfy their cravings without giving them a high, so that they can then function as normal members of society. I'm not aware of any evidence that methadone is any more dangerous than heroin, in any respect. There are of course dangers of overdose, but they exist for heroin too. In terms of secondary complcations (e.g. infections), methadone is virtually risk-free.