It's surprisingly hard! Granted, there's not usually any offense intended, but regardless, there's an implication of condemnation. Basically, you're calling them a sinner. I realized this when (long story short) some friends turned down board game playing on Sunday with the reason (in my understanding) that they didn't feel it was an appropriate Sabbath activity.
Of course they didn't intend to offend anyone, and I'm sure hinting that anyone who did was a sinner was the last thing on their minds. However, the implication was there, simply as an inherent factor of their statement of belief. I found it surprisingly hard not to grill them as to their rationale or basis for their belief, with of course the subconscious goal of proving that I was "right" in my beliefs and they were "wrong."
But the more I learn of how God really works, the more I start to realize that "right" and "wrong" really SHOULD be put in quotes. There's a reason Church leaders have avoided listing more than a general overview of "appropriate" Sabbath activities. There's a reason they don't define exactly what should be tithed. There's a reason Jesus didn't list off the exact nationalities that should be considered "neighbors" to the Jews in the parable of the Good Samaritan.
It all depends on your intent. If I think playing board games on Sunday harms the Spirit of the Sabbath (Monopoly, anyone?) but do it anyway, the Lord will judge it against me. If I think playing board games on Sunday builds stronger family relationships, then the Lord will judge in my favor. Exact same actions, but the intent was different.
...the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart.
1 Samuel 16:7
8 For behold, if a man being evil giveth a gift, he doeth it grudgingly; wherefore it is counted unto him the same as if he had retained the gift; wherefore he is counted evil before God.Double standard? Maybe, but one we create ourselves. As a matter-of-fact, it's a multi-billion level standard, one for each person.
9 And likewise also is it counted evil unto a man, if he shall pray and not with areal intent of heart; yea, and it profiteth him nothing, for God receiveth none such.
And that's why we're told to Judge Not. Only God knows everyone's real intent, and only He knows the proper standard to judge their actions by.