How can one verify the ritual impurity (najāsa) of water found on the road, or if impurity has overwhelmed it, and does this necessitate stopping to check its color and smell?

What splashes onto you from street water is excusable and pure, even if it contains some impurities. You are not required to investigate its purity. However, if you are certain that what touched you is the impurity itself, then you must wash it off. You must avoid obsessive doubts and not pay attention to them.