This is a hard one for the consumer. You can read more about which are 'safe' supplements at the government's website about it:#Information
. You could also joinhttp://consumerlab.com
which independently tests many supplements to make sure they contain what they say they do. If you are really motivated, you could also do your own medical literature search athttp://pubmed.gov
to see what studies have been done on a supplement. But probably the most practical approach is to find a practitioner you trust who can recommend supplements to you. Feel free to ask that practitioner why they think that company is a good one.
In terms of finding a good practitioner, that is equally difficult. Ask lots of others, including health care practitioners and patients, look at reviews on-line, see what kind of credentials they have, and then try them out.
There are definitely good supplements. Whether there are bad supplements is more difficult to answer. In general, I prefer supplements that are whole food-based rather than synthetically derived. The latter is much more like a medication than the former. But there can still be a role for synthetically-derived supplements for the person who is, for example, deficient in a certain nutrient or is getting a high dose of a supplement. The other thing to know is that a supplement may be 'bad' for you, but just right for the next person.