Usually made clinically
In severe cases, work-up includes basic labs and cultures
Punch biopsy: to confirm the diagnosis and to rule out other diagnoses (looks different from SJS/TEN histologically)
Erythema multiforme
Positive Nikolsky’s sign
Cold urticaria
Febrile
Cutaneous larva migrans
Staphylococcal scalded skin syndrome 
Herpetic whitlow
Toxic epidermal necrolysis/SJS 