If you see anything that is a difference or any type of clothing it is a Hidden Item, which you should always try and get those first, since some Hidden Items are hard to get in some mini-games.

Some differences in some mini-games are:
Squirrel Game: There should be 1 purple colored chip other than the blue colored chips

Milk Bottle Game: There should be 3 blue striped bottles and 3 green striped bottles at some point

Some Hidden Items in some mini-games are:
Dunk Tank Game: There should be a Green Sombrero at the top of the screen. Just hit it with the ball.

Dunk Tank Game: There should be a Scuba Diving Tank on the floor. Just hit it with the ball.