You might read the link below, but I always knew immediately when my little girl had a uti because she would want out very frequently. She would just go a little bit. I would catch a sample in a sterile cup first thing of a morning and immediately refrigerate it and take it in to the vet within an hour or two. They would check it and give me a prescription. There can be other causes, besides a uti, however.
http://b-naturals.com/Sep2004.php