Checking the composition is definitely the right approach, but I'd also suggest paying attention to the price. good-quality butter can't be cheap if it's the real deal. if the price seems suspiciously low - it's almost certainly either palm oil or some kind of spread. real butter made from decent cream costs noticeably more than spreads, and this isn't a marketing gimmick, it's just how the costs work out.
in stores i look for butter with an honest ingredient list - just cream, sometimes salt. these days a lot of manufacturers are making an effort because consumers have become more attentive. but yeah, you need to look carefully, because the label says "natural butter" in big letters, but then in tiny print in the ingredients you've already got palm oil listed. not all products are the same even within one store chain.
if you get totally fed up with searching - try looking for local butter straight from the producers or at farmers markets if you have them. there, as a rule, there are no secrets - you can see right away what and how it's made. i often buy it that way myself - i know the producer, i know what's in it. usually more expensive, but the quality is worth it.