Might be handy to know the Model Year.
Any check engine lights?
As to your original question, usually when an EGR valve is dirty, the EGR flow is restricted. On later MY Mustangs, this will cause a DTC.
The signs of excessive EGR flow are lack of power especially at part throttle. Use an ODB2 scanner to monitor DPFE flow. Or if high mileage, replace the EGR valve.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.