IF, your pool has little or no Cl, you can test the cell for production by just unscrewing the downstream union on the cell, enough to get it to leak, and take a sample of water for your Cl test.
The pump, and the unit are on of course.
Then compare it to your pool reading. This is the best place to check. I'm no chemist, but from what i understand, the Cl can be oxidized, out gassed, whatever, in the return line on the way to the pool. So testing right at the return for production may not be as accurate.