The most polite way to ask someone if they understand English is to use a considerate and respectful tone. You can say something like:
"Excuse me, do you happen to speak English?"
"Are you comfortable with English?"
"Can you understand English?"
"Would you like me to speak in English?"
"Is English okay for communication?"
It's essential to ask the question in a friendly and non-judgmental manner, making the person feel comfortable and at ease. If the person indicates that they do not understand English well or at all, try to find alternative ways to communicate, such as using simple language, gestures, or possibly seeking the assistance of a translator if necessary. Being patient and understanding will go a long way in fostering effective communication and creating a positive interaction.