I had a fungal infc. I used cortisone cream then I got a white patch, how can I know that if this vitiligo or just a side effect of the cream?

Hypopigmentation. After a skin inflammation area or dermatitis lesion due to any reason is resolved, it will look lighter than the surrounding normal skin area. It is likely due to suppression or death of the local pigmented cells in the process. The white patch is more pronounced if you have dark complexion or expose to sunlight. It will gradually fade away. It might take many months or longer.