If you have no testing devices you can mix up 1/4 strength solution and top with that. Once you have put in half the res again you should probably change the tank
IF water and nutes are not an issue, the absolute best would be to relace. You can never tell what nutes have been used even if you know the ppm.
But, most do not becuase of expense, hassle, possible over-kill. Just change every 7-10 days. Top with ph's water, and you should never have an issue.
Get a cheap ppm pen. That will tell you if your plant is drinking or eating more.