For me, the first sign is a dry mouth that stays dry after a couple sips of water. Another sign is twinges of cramps in my legs, when I know the level of effort doesn't warrant it. The latter is less reliable and harder to detect. Both of those can be corrected (at least for me) by popping a couple of extra Endurolyte tablets and maintaining the water intake. One other indicator is the saltiness of your sweat. Lick your lips.. if you taste salt, then you are likely ok.