When is 25 modifier used?

As per the CPT definition, Modifier 25 should be used along the E/M code when a separately identifiable CPT service is done on the same day by the same physician.                                                                        

The thumb rule is whenever there is a CPT code on the UB form and its status indicator is S or T, then modifier 25 should be appended to E/M code.                                                                                                           
Some of the common procedure that will require use of modifier 25 to the E/M code are:
Any CT, MRI, or Ultrasound,
EKG
Nebulizer Treatments
Infusions or injections

So if you find any of these on your chart you can append modifier 25 to the E/M code.