Slow leaks can be tough to catch during a preflight inspection. Most liquid filled containers are hidden behind the engine cowl or wheel wells. But if you pay attention to changes on the ground you can catch small leaks before they become major trouble.
When you walk around the airplane, look at the ground underneath. Any stains are potential signs of trouble. It could be an oil leak under the nose or perhaps a hydraulic fluid leak beneath one of the main wheels.
The color of the stains will tell you what system they come from. Hydraulic fluid is red, so any red stains would likely come from brake systems or perhaps a landing gear system. Other systems can be hydraulically actuated as well. You should know your airplane’s systems intimately.
Fuel leaks are harder to see on the ground since tarmac is often very dark and fuel stains are either clear or light blue. Fuel leaks are best detected by looking at the underside of the wings for blue stains.
It is more likely that the stains on the ground are dark and sticky, indicating some type of oil leak. A small stain is not generally a cause for alarm. But it should be monitored and if you see the stain grow with time you should have a mechanic take a closer look at your engine.
You can also find leaks in the propeller governor by inspecting the propeller blades. If there are any signs of grease on the blades there is likely a leak somewhere in the system.
And while you are taking a closer look at the ground you may also find a bolt or nut that has fallen off the airplane. A few missing bolts could cause a fairing or inspection plate to start shaking, which could damage the piece or even make it fall off the airplane.