Most stuff works, but I usually go with a fork. [Unless the forks are all dirty, in which case he will use a spoon, or a giant serving spoon, or a spatula, or attempt to balance it on a knife... Sans won't actually object if Alfred just grabs some in the most literal sense, but he will point to a drawer also.] Forks're in there.

[As for why not to take food from alarmingly-happy strangers...]

The stuff the people who're from here give you has stuff like razor blades in it sometimes.