I ordered a few sets of Descent after I watched a friend play and after opening a few boxes I mixed up the cards. Now is there a way to unmix them? I don't remember which cards went to which expansion... I'm not sure if it matters that they are mixed up, but it seems like it does.
I've not handled Descent cards myself, but in my experience most games with card-based expansions — Fantasy Flight games in particular — tend to have small icons on their cards which indicate which expansion they come from; I usually find this in the lower right corner of the card itself (no icon typically means it comes from the base set).
No, it doesn't matter if cards are mixed up. The game is designed to be mixed and matches at will. You can play any subclass in any expansion. You can buy any item at the shop from any expansion as well.
Only overlord cards are played "face down". Everything else is displayed on sight, and marked exactly when to use. For example, act 2 shop items are marked with a "II". If more expansions are mixed in, that just means you'll have more loot to choose from in Act II. There are some scenarios (tutorial for example) that will tell you exactly what to bring to the table. Having more cards, just means more options for all the players.
Cards are marked with their expansion symbol, if for some reason you want to check them individually or remove them from the game.
A side comment, I'd check the app if you haven't yet. It basically plays the Overlord (which almost no one likes to play), and you and your friend just play the heroes. The app is called descent road to legend, and it's available on Steam, iOS and Android for free. It uses every expansion you have (you select them in an option), although only in side quests, and minions. They just launched The Delve, a campaign that'll mix and match content from all your expansions, but it's payed for (5$). I'd try the general campaign first, and then if you enjoyed it, go for the Delve (which is much more re-playable).
While some FFG games like Arkham Horror function best with just one or two expansions and can become unwieldy with more, Descent just gets better and better with each expansion added, just like @CyberClaw mentioned. And as @goldPseudo mentioned, there are typically small icons on the card to indicate the set each card belongs to.
For more information on those icons and a quick reference to which set they belong to, the Descent Wiki - Expansions page can be extremely useful, as each expansion's detail page lists not only the detailed contents of the expansion, but also the set icon for that expansion.