In the top let corner is a triangle with curved side only on stacked items, you can try to compare the icon to a single item(stacked items will have a # on bottom left of icon).

 if you hate having a bunch of cake leftovers in fridge you can use the testingcheatsenabled true cheat and shift click the sim you want to age manually, w/o the party, good if u have like triplets+twins, and will be their all day celebrating the parties.

in the corner of the icon on multiple item stacks, the part where u click to expand the item, click and hold and drag to container u wish, it will put multiple items down at one time.

testingcheatsenabled true

this allows you to shift click mailbox to max ur needs, make them static, and choose career(note: using the career option won't complete your lifetime wish).  Also you can shift click sim and have them grow up.

Forgot to also add you can have your sim selected and anywhere in neighborhood you can shift click ground and have them teleport their, nice when ur out collecting.