As a part of a larger job, I have a plumber replacing water lines. He is going to use PEX. I read that sunlight exposure causes PEX to become brittle very quickly (on the order of a couple of weeks). Based on the I-know-it-all attitude that the plumber has showed on other parts of the project, I am worried that he might have stored the pipe improperly. Is there a way for me to check the state of the pipe before the walls are closed?
Most water PEX pipe has a UV stable outer coating.
Check the brand and spec sheets. If it is UV stable, its fine unless the sheath is cut or damaged.
NOTE: gas PEX does not have this coating.