Here are some signs that L style narrow crown staples may be loosening:
Visible gaps around staples
Small gaps where the staple meets the material indicate it is pulling away.
Easy to move by hand
If staples can be wiggled or pivoted then they have lost securing force.
Material movement
Over time, materials attached by loosening staples may slip or sag away from their positions.
Audible movement
Gentle tapping or scraping near staples may produce a subtle clicking sound as they shift.
Vibration or noise
Frequently used structures may develop new creaks and groans from loose staple joints.
Rust stains
Dark rust streaks radiating from staples show corrosion accelerating as water gets behind them.
Squishy feel
Pressing near staples in spongy materials feels squishy rather than rigid if securing has slackened.
Pull testing
Gently tugging perpendicular to grain on materials should not result in any movement around staples.
Regular inspections, especially after weather extremes, catch loosening L-staples before failure of the assembly or installation.