Pattern
Notice whether the problem affects old or new leaves, tips, margins, spots or one side of the plant.
Houseplant problems
The symptom is where the investigation starts.
Brown tips, soft leaves, spots and slow growth do not identify one cause on their own. The useful question is what the symptom looks like in context: water, light, roots, potting mix, temperature, pests and recent changes.
Before treatment
Describe what you see, then narrow the cause.
The same visible change can develop for very different reasons. A wilted plant, for example, can have an extremely dry root ball or damaged roots sitting in wet mix. These guides therefore begin with observation and eliminate possibilities instead of prescribing one automatic remedy.
If decline is rapid, widespread, associated with extensive rot or a serious pest problem, or involves a particularly valuable plant, direct assessment by an experienced grower or qualified horticultural professional can be more appropriate than an online diagnosis.
Roots
Check moisture, drainage and root condition when symptoms persist or contradict the surface appearance.
Timeline
Connect the symptom to moving, repotting, watering, heat or seasonal changes.
Observe. Care. Remember.
A timeline makes symptoms easier to understand.
A photo, the repotting date, a location change or a different watering pattern can explain what otherwise seems to have appeared from nowhere.