Suspension components are the most variable on a bike – each manufacturer has a different way of building and maintaining their forks and rear shocks. This makes it hard to give specific advice, but there are some common elements to all systems.

Fork loses pressure
The ride gets harsher, or the front of the bike seems lower than normal.
Rear shock loses pressure
The bike sags more than you expect, bumps are harsher, and pedalling gets harder.
Shock bolt breaks
Although it sounds catastrophic, you may not realize the bolt is broken because the rest of the bearing/bushing pieces may hold it in. A “noodly” feeling frame is one indication.