So because you used Nitrox on a previous dive and are less tired you can somehow tell you're using Nitrox on a consecutive dive? I don't think so. Regardless of how many dives you do or don't do on Nitrox, you can not tell, on any individual dive, what gas you're breathing without analyzing the tank.
That's the point of the whole side discussion. You can't tell what gas you're breathing just by breathing it so there isn't any need to do a dive in order to get the Nitrox cert. It feels the same as breathing air and the only practical knowledge of the course is how to analyze a tank.
Im just going to agree to disagree... You have far more posting experience then l. Cheers