ClO2 chlorine dioxide how to know if it is still active or not ? by color ? by smell ? by reaction?
If you start with $\pu{100 mL}$ stock solution at $\pu{100 ppm}$ concentration, and dilute this by a factor of 10 to yield $\pu{1 L}$, then you are correct, the later one has a concentration of $\pu{10 ppm}$.  Keep both in a cool place.
As to check if the solution still is active, or not, and both stock and diluted solution at the expected concentration (which will decrease already if the solution is exposed to hot summer temperatures), you may use test strips you immerse dip into the solution, dry at room temperature, and compare against a scale.  An example were the following

(source)
Depending on the manufacturer, the scale of concentration to test may vary.