Metalcore bands often play with melo-death structures or riffs nowadays, it's kind of a trend I guess. They throw in the notorious hardcore breakdown which usually consists of chugging guitar lines with lots of deadspace between notes and a much more abrupt transition than breakdowns found in other genres, esp. death metal.
Melo-death bands are like old/mid-period Soilwork, Dark Tranquillity, Cipher System, etc. who may use these breakdowns here and there but not to the same effect as a totally "metalcore" band. I agree, sometimes it tends to get confusing as recently there's been a lot of crossover with these two genres and a lot of bands that once you could tell were total "melo-death" in the original style have been peppering in metalcore breakdowns...but, I digress. Essentially, you just need to listen to a lot of stuff and decide from there.