Compare equal amounts
A tub price can hide a big difference in how much you get. The creatine guide works through two sample containers at the same price.
What was tested?
Check who tested the product and what they measured. Quality testing can cover ingredients or contaminants; a seal cannot guarantee a supplement is safe or effective for you.
Sources: NIH: Supplement basics
Flavor and preparation count too
Choose a flavor and preparation you actually like. Unflavored, sweetened and chewable products offer different options; none of those preferences proves a health claim.