Disclaimer: This riddle MAY have been given elsewhere. I heard this one a few months ago from a friend, and I have no idea what his source was.
You're in a room, since that's your job. Your singular apparatus, is of course, a bucket of sand. It is very important to you, and indeed, vital to your job. You know exactly how many grains of sand are in your bucket, and are very proud of this fact.
However, one day, tragedy strikes and a absent-minded secretary spills her mug of spaghetti sauce into your bucket! After much mourning, you pour out the tomato-basil flavoured mess in your bucket, and replace the sand. Distressingly, you realize that you don't know how many grains of sand are in your bucket anymore!
But, as luck would have it, a man comes and claims he knows exactly how many grains of sand are in the bucket. Indeed, he boastfully claims he can tell you the exact number of grains of sand particles in any vessel at a glance, and happily offers his services.
However, due to legal issues, he claims he cannot tell you the number of sand particles in your bucket before you hire him.
How can you confirm that the man knows exactly how many sand particles are in your bucket or if he is simply a charlatan seeking a quick cash grab at the heels of a tragic event?