From datasheet:
"On first application of power to the device the time and date registers are typically reset to 01/01/00 01 00:00:00 (MM/DD/YY DOW HH:MM:SS). The CH bit in the seconds register will be set to a 1."
Check registers and if date is 01/01/00 and the time less than 00:00:10 (or something similar depending how long your code is running before checking) then ds1307 not setup?