When I assess a supplement, I look beyond the advertising or the manufacturer’s promises. Essentially, I’m looking for the answer to two questions: does the active ingredient actually have a scientific basis, and is the specific product good quality? For that, I use two different, independent resources.
To check whether an ingredient has a scientific basis, I use Examine — an independent, evidence-based database. It evaluates not isolated studies that happen to support the marketing, but the whole body of scientific evidence on a given ingredient and its effect on a particular condition. Some of the content is free, while the fuller database is available with a paid Examine+ subscription. Importantly, Examine has no advertising or industry sponsors — it is funded entirely by its readers.
To check whether a specific product is good quality, I use ConsumerLab (I have a paid annual subscription). This independent organisation laboratory-tests a selection of the supplements on the market and evaluates whether they actually contain the amount of active ingredient stated on the label, whether they are free of certain contaminants, and whether the product meets their quality criteria.
Important: neither Examine nor ConsumerLab covers every ingredient and product, so the absence of something from their database does not by itself mean the ingredient or product is poor. They are simply two of the more reliable resources that help me choose based on more than just marketing.