Describe the approach.
Simple febrile seizures do not require specific diagnostic workup
Diagnostic examination focuses on identifying the cause of fever
Complex febrile seizures always require specific investigative tests (e.g., EEG and imaging).
Describe the further evaluation.
Determine the cause of fever
Urinalysis and urine cultures
Consider blood tests (CBC with differential blood, CRP, electrolytes, blood glucose)
Consider imaging to locate the source of infection (ultrasound, X-ray)
In suspected cases of intoxication: toxicology screen
Additional diagnostic steps: for complex febrile seizures and/or abnormal neurological examination
To exclude meningitis and/or (herpes) encephalitis 
Imaging (e.g., CT or MRI)