I don't need to have early check in usually, but when I do I just ask on arrival since they mentioned it's based on availability anyway and I doubt they will confirm in advance by writing.

If there is online check in and I will arrive early I do tend to do that too also, as there is usually a field you can fill your arrival time. Not sure if it helps so much but I usually get it both when I'm gold or platinum, although as I said I only need it sometimes.