You will know if it goes to your kidneys. A kidney infection causes really bad pain in your flank area and you will feel sick! Don't let a UTI go, take your antibiotics that will kill the infection. Cranberry juice and lots of water helps to flush your system. Make sure to drink plenty of fluids.
Cranberry juice can be good preventative for UTI, but may not be enough for infection. It is good to get antibiotics for UTI but does not guaranty it will not progress to kidneys.