Toofer, Keeper of the Full Grip

When you cast Divination from your hand, you’re losing a card (Divination itself), but after it resolves, you’ve gained two cards in your hand. Overall, you’ve gained a card. Opponents haven’t gained any cards, so you’ve gained card advantage. Conversely, when you cast Prosperity for X=4, you lose a card (Prosperity itself), but you gain four cards, for a net gain of three. However, even with just one opponent, that player will draw four cards, so that ends up being a total net loss of one card for you, which means Prosperity with X=4 was not card advantage.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Abilities that cause you to gain card advantage won’t cause Toofer’s ability to trigger, even if those abilities trigger from casting spells. Similarly, card advantage from other sources, like combats where opponents lose more creatures than you do, won’t cause Toofer’s ability to trigger.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

Card advantage is determined based on what actually happens in game. For example, Annihilate will be card advantage if it resolves normally targeting a creature an opponent controls—you lose one card from your hand but you also draw a card, for a net gain of zero cards, and your opponent loses one card, so you’re up one card relative to them. However, if the target creature has indestructible, you and your opponent both have a net gain of zero cards, so that’s not card advantage.