Simplest finesses in Spades?

I think this completes the other answers, implementing the idea into actions.

Indirect Finesses are when partner plays a non-honor card towards your tenace. They also sometimes occur when you are second to play in the trick. You will be the one to win the trick if successful.

1. AQ, AQJ, AQJ10 - 50% chance to score all tricks. 50% chance to score all but one tricks. Virtually always finesse East for the K regardless of who leads what.
2. AJ10 - 75% chance to score all but one trick. 25% chance to score one trick. If East holds the K, Q, or both, you will score all but one trick. Finesse from second seat or if partner is denying strength with his lead. Don't take this finesse if partner is promising an honor with his lead.
3. AQ10 - 25% chance to score three tricks. 50% chance to score two tricks. 25% to score one trick. Play the 10 from second seat or from third seat if partner has conveyed shortness in the suit. If East holds both K and J, you will score all three. If East holds only K or J, you will score two. You only get robbed of additional tricks if West holds both K and J.
4. AKJ, AKJ10 - 50% chance to score all tricks. 50% chance to score all but one trick. Do not finesse on the first lead of this suit so you can gather partner's attitude towards the suit. Only finesse East for the Q if partner has shown shortness in the suit. Otherwise, it is best to hope for the Q to drop on your AK.
5. KQ10 - Hope for doubleton or honor from partner instead of finessing. Consider leading from this. If the suit gets led by partner or East, finesse East for the A, not the J.
6. KJ, KJ10, KJx or longer - 50% for one trick, 50% for no tricks. If East leads the suit and you hold KJ or longer, play your lowest card. If West has AQ, he is going to finesse you anyways.

Source: https://www.reddit.com/r/spades/comments/f34h7u/common_tenaces_and_finesses/

Finesses are all about considering the risk. In the example Benjamin notes, where partner plays low, right-hand opponent (RHO) plays low, and you hold AQ, you have a choice. You can play Q, which has a 67% chance of winning (with no other information), or you can play A, which has a 100% chance of winning but promotes the opponents' K to a trick 33% of the time. Sometimes one or the other is the right choice: you have to decide how important it is for you to be on lead.

Starting a finesse for sure is difficult in spades, but the idea of the finesse is still something that should direct your leads to some extent. For example, you should avoid leading away from AQ - that means you'll never get that finesse, and opponent holding the K will likely win it even if it's RHO.

Further, information is available: one round of bidding exists, and so if one of the opponents indicated a higher bid, then guess what - they probably have most of the power (or most of the spades, but hopefully that becomes evident sooner than later). If that's RHO, then be very cautious both leading away from trapping honors (AQ, KJ) and just leading away from honors in general - this is something you always should avoid, but avoid it more. Conversely, if LHO is the stronger one apparently from the bidding, you might be more inclined than normal to lead away from strength - in the hopes that you draw out one of LHO's trapping honors and promote yours to a trick. Lack of information cuts both ways, after all!

The smaller amount of information in Spades means it is hard to purposefully start a finesse, but you can still easily continue one if it is started by accident. For example, your partner happens to lead small and first opponent plays small, and you are holding the AQ. You can now play the Q and hope that the K is not in the remaining opponent's hand. In Bridge in the simplest model, this has a half chance of working because you don't know which opponent has your missing K. Here in the simplest model it's very similar: there's a 1/3 chance your choice doesn't matter at all (because your partner has the K), but when deciding your move you should assume that your choice does matter (such an assumption will never hurt you), and so there's effectively still a half-half chance of success.