The simplest way to tell if you’re hydrated is to monitor the color of your urine.
If your urine is pale like lemonade, odds are that you’re drinking enough. If your urine is dark like apple juice, then you should probably be drinking a little bit more.
Keep in mind that if you take a multivitamin, the soluble vitamins in your capsule (or other delivery form) may cause your urine to turn bright yellow in the hours after you take it. This should not be confused for being dehydrated.