Moisture stains on the ceiling: how can I tell if it's mold or condensation?

I had the same problem at home, and the main difference is that condensation disappears when you dry the surface, whereas mold stays and always comes back in the same spot. Try cleaning it thoroughly with a damp cloth: if the stain comes back after a few days, it's mold and there's a ventilation or insulation issue that needs to be fixed (maybe a small leak from the window or moisture rising up through the walls).

The difference they mention in the other answer is right, but really it's not always that clear-cut. If you dry it and the stain comes back in the same spot within a few days, yeah, it's mold and not just condensation. You mainly see real condensation when it's cold, especially in winter, and usually it shows up on smooth surfaces like glass. What you've got though, especially in a corner near the window in summer, smells more like water seeping in or stagnant moisture that's created the perfect breeding ground for mold. Just cleaning won't cut it if there's an underlying structural issue with insulation or air circulation - it'll come back eventually. It'd make sense to check whether cold air is getting in through the window on that side or if there's poor insulation, before you decide whether it's worth calling someone in.

The most reliable test is to touch the stain: if it's cold and damp to the touch while the rest of the ceiling is dry, it's probably condensation. But if it's dry and has a powdery consistency or is slightly sticky even when there's no visible dripping, it's almost certainly mold. Another telltale sign is the smell: mold has that characteristic stale, musty odor, while condensation doesn't smell.

In my case I had both things happening at the same time in my bathroom - condensation on the window but also mold in the ceiling corners. The condensation would disappear when I turned on the fan, while the mold came back within a week even after I cleaned and dried everything thoroughly. The difference is that condensation is a symptom of humid air (poor air circulation, temperatures too low), while mold indicates that the humidity has already created conditions for biological growth - in other words, the damage goes deeper.

In your case, since it's always in the same corner near the window, I'd say it could be a combination of both. Try cleaning thoroughly with a solution of water and vinegar (you don't need bleach on all surfaces), dry everything, and then monitor it: if it comes back quickly in the same spot after a week, it's real mold and there could be a water leak or an infiltration from the window, so yeah, it's worth having someone who knows what they're doing take a look at it. If it stays dry for weeks, it's probably just a matter of better ventilation in the room.

When you first noticed the stain, was it wet to the touch or already dry?

The distinction that the others explained works, but I'll add a practical detail: mold usually has a well-defined gray-black halo and a slightly grainy texture if you touch it up close, while simple condensation tends to be more uniform and transparent. In your case, since it's always in the same corner near the window (cold spot) and the summer was humid, it could be condensation if the room doesn't have ventilation, but the fact that it's been there for a month without disappearing is suspicious.

Try this: open the window wide for at least an hour and then see if the stain changes. If the gray becomes less visible, it was mainly condensation from moisture buildup. If it stays the same, it's real mold. In the second case, the real problem isn't the stain itself but what caused it - leaks from the external wall, poor air circulation, or rising damp. That's when a technician could help you figure out the cause. If it's just condensation, though, you just need to improve air circulation: open the window regularly, especially when you're cooking or showering, and make sure there aren't cold drafts that increase the temperature difference.

What you should do right away is check if the area around the stain is colder than the rest of the ceiling - touch the wall nearby with the back of your hand and you'll feel the difference if it's pure condensation. In my apartment I had exactly the same situation in the bedroom, and the corner near the window was always freezing when rain and humidity hit, while the rest of the room was normal. If it's really just condensation, increasing ventilation (opening the window even if just for a bit, maybe in the evening) and putting a dehumidifier does wonders in a week.

But if after a week of frequent ventilation the stain doesn't fade and starts to have that characteristic stale smell, then there's an insulation problem or water infiltration underneath, and in that case you need a technician. Don't wait too long because real mold spreads slowly but steadily.