5)Look at independent sites such as www.consumerlab.com for reviews, or check to see if the company USP or NF seal (US Pharmacopoeia or National Formulary standards for that product). If it’s GMP (or Good Manufacturing Practice), then the actual facility has been checked out and approved. GMP does not look at quality of the actual supplement.
I know there are hundreds, if not thousands, of companies out there. Talk with a qualified practitioner who works with nutritional supplements for guidance and really do your homework. Your health will thank you in the end.