1) the context (by reading the whole story you will know that this man does not appreciate Asterix and Obelix)
2) the wordily content (“Garedunord, it is us”)
3) the evil facial expressions of Asterix and Obelix suggesting irony and malice
4) the speech bubble flowers indicating waking the sleeping man up in an overly not-so-nice way.
WOW.
The wordily content alone would not have made us realized the meaning of sarcasm, but in combination with the others - stuff we know as means for showing sarcasm in the real world, it guides our reading of the image.
Though, someone else could have interpreted this differently…? How we read the communicational aspects can vary from person to person, culture to culture. I base my readings of images on a social semiotic framwork, theories that help me analyze graphics and typography as resources for producing meanings.