They can confirm you the number on the certificate is real (ex : X3L 654 = canadian 10c 1932 in Ms-64) but they wont be able to tell you if the coin itself is the one they paired with the certificate to begin with.
''Buy the very best, stretch to buy it. It means if you can't afford to buy it, buy it anyway."
While there are some ICCS holders out there that have been tampered with, the vast majority--probably 99.9%--are what they are supposed to be.
Unless you have a reason to be worried about your specific coins (you bought them off of someone knowledgable for a silly price; coin appears to be a forgery; something is horribly wrong with the grading), I would not spend time on this.