Many worries about a vehicle begin as a sensation: a light, a sound, a smell, a shake. A clear written description lets you decide whether the thing you noticed is in the owner's manual, whether it is normal for the vehicle, or whether it needs a shop to look at it. Write the description before you ask anyone what it means.
Describe the sign itself
For a dashboard light, record its color, whether it is steady or flashing, and whether it stays on or comes and goes. For a sound, note where it seems to come from, its pitch and rhythm, and whether it happens on acceleration, braking, turning, or at a steady speed. For a smell or a spot under the vehicle, note the smell, the color of any fluid, and where on the ground it was found.
Note the conditions when it happens
Write down what the vehicle was doing and the outside conditions: a cold start, a warm engine, a load in the back, a trailer, a visit to a shop, or the weather. Add the odometer reading. A symptom that appears only under certain conditions is easier to match to a cause than one that seems to appear at random.
Check whether it is normal
Some sounds and lights are part of normal operation. Look in the owner's manual's section on warning lights and on the systems you suspect. The manual may describe a light and the step it asks you to take. If it does, follow that step. Do not decide the cause from the color alone.
Take the description to the right source
If the manual does not explain what you saw, bring your written description to a shop and ask the shop to repeat the symptom back to you, to note the conditions you recorded, and to write its findings down. Keep your description with the paperwork so you can compare it with what the shop found.