We have to watch DS2 all the time for UTI's as he has a kidney problem. Signs for us are:

High Temp
Lethargic
Strong urine smell
Drinking more than usual
Sometimes vomiting

It is hard in toddlers to know for sure so only a sample can rule it out but it's something I'm extra vigilant with as I had 18yrs of UTI's before it was discovered one of my kidneys is irreparably damaged and now only works at 10% of its normal function.