That mark on the ceiling you keep glancing at
You noticed it after the last rain. A brownish ring on the ceiling near the bathroom, or a dark smudge in the corner of a closet that wasn't there last summer. You've probably stood under it with a flashlight more than once, trying to decide if it's getting bigger or if you're just paying more attention to it now. That uncertainty is normal. Los Angeles homes spend most of the year bone dry, which means when water does show up, the evidence sits there a long time before anyone deals with it.
Most people in this situation aren't actually asking "is this mold." They're asking "do I need to do something about this, or can I paint over it and move on." That's the right question. It's just not always an easy one to answer by eye.
Why older LA housing shows marks like this at all
A lot of the housing stock here, the 1920s Spanish-style bungalows in places like Highland Park, the 1950s stucco ranch houses spread across the Valley, the post-war tract homes in South Bay, was built for a dry climate and hasn't been seriously updated in the ways that matter for moisture. Flat or low-slope roofs common on mid-century homes are notorious for slow leaks that don't show up until the one or two heavy rain weeks we get each winter. Galvanized plumbing in older houses corrodes from the inside and can weep slowly into a wall cavity for months before a stain ever reaches the surface.
Add in the fact that many of these houses were never built with much ventilation in bathrooms or laundry areas, because nobody in 1955 was worried about humidity in a desert city, and you get a pattern: moisture gets in, has nowhere to go, and leaves a mark on drywall or plaster that looks alarming but might be nothing more than a one-time event from a roof leak that's since been patched.
More on this from Reading List The Difference Between A Stain And A Problem In Los Angeles Housing.
What a stain actually is versus what a problem is
A stain is history. It's a record of water that was there once. If the source has been fixed, the wall is dry to the touch, and the mark hasn't changed shape or color in weeks, you're probably looking at residue, not an active issue. Old water stains are often rings with a hard edge, sometimes with a faint yellow or coppery tint from minerals in the water.
A problem is ongoing. That means the material behind the stain is still wet, or wets again every time it rains, or the air around it smells musty even when the room is closed up. Mold needs a continuing moisture source to keep growing. No moisture, no growth, no matter how bad the stain looks. The visual mark and the actual condition underneath are two different things, and that's exactly where people get it wrong in both directions: panicking over an old dry stain, or ignoring a damp patch because it's small.
What you can reasonably check yourself
Press on the area. If the drywall or plaster feels soft, spongy, or cool and damp compared to the surrounding wall, there's moisture present right now. A dry, firm surface is a good sign. Check if the mark has grown or darkened over the past month. Take a photo on your phone and compare in a few weeks if you're not sure.
Look for the obvious local causes first. In LA that usually means a roof that hasn't been checked since the last good rain year, a cracked tile grout line in a shower that backs onto the stained wall, an AC condensate line that drains improperly in the attic, or a planter box or irrigation line right outside that's been overwatering the base of an exterior wall. These are things you can trace yourself with a flashlight and twenty minutes, especially if you have attic or crawlspace access.
Where it stops being a homeowner job
If the stain is damp to the touch, if it's spread past a dinner-plate size, if you can smell something musty in that room with the door shut for a day, or if it's located somewhere you can't easily see or reach, like inside a wall cavity around old cast iron plumbing, that's the point to bring in a mold inspection rather than guess. Testing and moisture meters can tell you things a flashlight can't: whether the material is still actively wet, how far the moisture has traveled inside the wall, and whether what's growing is a surface mold that wipes off or something rooted deeper in the drywall paper.
This matters more in certain LA housing than people expect. Homes with original plaster walls hide moisture differently than modern drywall does, and older additions bolted onto a house without matching ventilation often have their own hidden wet spots that have nothing to do with the stain you're looking at. An inspection gives you a map of the house's actual moisture condition instead of a guess based on one visible mark, and it tells you whether you're dealing with a repair job or a remediation job, which are very different in scope and cost.