No more u/s - how to know if fluid is low?

FirstTimePregnancy

I saw my OB today and confirmed that they do not plan to do any more u/s unless there is cause for concern.  I have read numerous posts on here about women having low fluid levels and am wondering if no more u/s are done, how do they know if the fluid level is safe?  Obviously if you start leaking then they will check on it, but I would just feel much better if they planned to do another u/s to be sure all was OK.