100% antifreeze doesn't have any adverse effects that I know of. If you really want to know, do as the others said and pick up a hydrometer.
Or, you can go back and ask his/her manager to come out to your car with a hydrometer to make sure it's within a safe range. I know when I worked at Valvoline in Kansas City, we had to put down what the measurement was on every sheet that got a radiator flush, even though our coolant came pre-mixed in 55gal drums. To be honest though, we always just wrote 50/50.
