“I can only try”, If you only eat Cholov Yisroel, and the person you eat by eats Cholov Akum, he may have used his keilim to cook Cholov Akim food. So all his keilim may potentially be unusable by you, even if he honestly serves you only food that you would rely on.
Or similarly if you only rely on certain hechsheirim for meat, and he cooked with his keilim (long before your visit) meat with hechsheirim you wouldn’t use, all his keilim may now be affected by that non-reliable (for you) hechsheirim.