That over-the-range microwave above your stove usually has a vent on the back — but in a lot of homes it doesn’t go outside at all. It just recirculates: the air passes through a filter and blows right back into the kitchen. For heavy stir-frying, grease and smoke end up back in the room, staining walls and lingering as odor.

Here’s how to tell in three quick checks. First, go outside and look for the matching vent cap on the wall or roof — a real one has a flap that opens when the fan runs. Second, look behind and above the microwave: a duct turning up and out means it’s vented, while a sealed back panel with a top grille blowing inward means it’s recirculating. Third, the easiest one — check for a charcoal filter; recirculating units use one, and truly vented units don’t.

Too lazy to open it up? Run the fan on high and hold your hand over the top front grille. If warm, smelly air blows back at you, it’s recirculating. If the cap outside moves and nothing blows back inside, it’s vented.

One common trap: that aluminum plate on the outside wall might be sealed shut or purely decorative. Don’t assume a cover means it’s connected — a truly vented cap has a flap that opens when the fan is running. And if you’re planning to switch to a high-power Chinese-style side-draft hood, confirm you have real outside venting first; that’s what lets the strong suction actually work.

More Greater Boston kitchen and remodel work: see our recent projects.

Is yours vented or recirculating? Run the three checks and tell us in the comments.