ok, these come to mind off the top of my head (I had anorexia in my late teens):
hiding food, e.g. secretely shovelling food into a napkin and then binning it
saying they've already eaten when out (at school, friend's house, whatever)
spreading leftover food out on the plate to make it seem like less than it actually is
or piling the plate full of lettuce leaves so that it looks like a huge portion but really contains only 40 calories
cooking for other people (excessively - I used to bake elaborate cakes all the time but wouldn't eat any of it)
exercising obsessively - often in secret so I guess hard to tell
drinking things like marmite, bovril, those 2 calorie soup sachets etc. all the time
talking about food a lot, being unusually interested in food