How to test the coolant on a Citroen C3
1. Ensure the engine is cool or even better, cold. Do not try to remove the coolant cap on a hot engine.
Remove the cap and use the tool to draw up some coolant into it.
2. Draw coolant into the gauge by squeezing the bulb.
I had to do this in two passes by fully squeezing the bulb to draw up half the gauge, inverting the gauge so all the liquid runs into it. Gently squeezing the bulb with the tool upside down keeping the coolant in the gauge. Then took a second draw up to the fill line.
The fill line may not be easy to see, so I have marked it as a white line in the picture. You can see this one is a little over the line

Squeeze out any excess to bring it to the line or add more by inverting the tool and gently squeezing the bulb and drawing more coolant.
I had a few air bubbles on the gauge needle so I tapped the tool body gently to remove them and let the needle settle. With the gauge set vertical (by reading the top needle), a reading can be taken from the scale.
If your coolant isn't a nice clear pink colour or doesn't offer the level of protection you require (at least a recommend -35°C) it could be time to replace it with this OAT coolant for the C3 (here)
Pick up your own tester here (Amazon) or here (eBay).