True, Presbyterians are never going to hold joint services with Catholics for that reason, and vice versa. When it’s just an individual though it can be a little more difficult to determine what they really believe which is why some churches treat them suspiciously.
I concur with you except for the 'suspiciously'. I think churches have a duty to address those non-members prior to communing them. And it is out of love for them and love for God. Once done, it is an individual responsibility as no examination can read the heart, as humans we must trust their words.