The most obvious sign is often an increase in water consumption and this seems to increase as the condition progresses but there are other symptoms you can look out for such as a loss of condition, change in temperament and often diabetics will have a definite wet corner/patch when they may not have done so before. The urine can feel a bit sticky and does have a definite scent to it.
There's a wonderful article on diabetes on Vectis Hamstery's website, it's written primarily with Chinese in mind but a lot of the advice applies to other species too:
https://vectishams.com/chinese-hamster-diabetes/