Is what the questioner does correct, changing clothes and showering upon returning home due to splashes of sewage water or unknown puddles on his body and clothes while riding a motorcycle in India, and how can one ascertain purity in this case?

It is not obligatory to wash whatever splashes onto the foot or garment from street water mixed with impurity, unless the impurity is غالبة (predominant) over it or its contact with the garment is certain. It is stated in "Al-Taj wa Al-Iklil" from "Al-Mudawwana" that there is no harm in rainwater mud mixed with impurities in the streets if it splashes onto the garment or body, as long as the impurity is not غالبة (predominant) or its عين (substance/actual presence) is not apparent.