The only way to tell is the smell but the fresher the better. I would never use tea that is more then a day old. The only reason to do this would be if you had multiple acres and could not get it all out on the first day. Also, if you are making supplemental tea then I would dilute it 4 tablespoons to a gallon. If it is a feeder tea then you can pour it straight or dilute 1:1.
I prefer a supplemental tea 4 tablespoons to a gallon so that I can get in more feedings and keep the organic buildup as low as possible at the beginning of my grow.