My DS had this at 7 weeks. We found out because he had a temp. we were admitted to hospital and they discovered a UTI. found kidney prob with an u/s.
I think temp is the main giveaway, (or recurrent UTI"s, which has temps with them.). Would do urine test first to see if there's a prob, then go from there.
Other signs are holding wee in and not urinating often. Also pain when weeing (these dont all have to be present, often none of them are).
xo