Well, there is coolant in there! lol. I can see the head of some sort of contraption in the middle, but i'd say it was 40% full.
EDIT: I'm basically after confirmation that if I top it up with blue, I'm not going to do any great deal of damage. It's not pink stuff. If the sensor has gone, I'll get it changed and it'll need new coolant anyway. If it goes off, I'll get some pink stuff for next time.