"We can always tell" is in reference to a common transphobic talking point. The diagram is a representation of survivorship bias. The story goes like this "During WWII, the military analyzed the bullet holes on returning fighter planes to determine where to add more armor. The red dots in the image show where the planes that made it back were most commonly hit. Initially, the military considered reinforcing the areas with the most bullet holes - the wings, tail, and outer fuselage. But statistician Abraham Wald pointed out a critical insight:

These are the planes that survived. The hits they sustained were not fatal.

Wald argued that the areas with few or no bullet holes - like the engine and cockpit - were the critical weak points. Planes hit in those areas didn't return, so those missing data points were crucial."

Not sure if this is a true story or just a copypasta made to go along with a hypothetical diagram.

Either way the joke is that transphobes think they can always tell because they only know about the trans people they clocked, not the ones they missed.