Many of the casino employees get most of their salary from tips. That is how it is for those who work in the service industry.
In slot machines, a tip is expected by the attendants when a slot player wins a jackpot. The slot attendant is the one going to handle the jackpot money, verify the jackpot, count the money, and then give the money to the player.
Usually there are two or three people who are there in attendance for the hand pay. This is just for security reason to make sure the correct amount is given to the right person. Now the question is which one of these people should be given a tip.
You can just give one of them a good amount to them and tell them to split it amongst themselves. In this case give the tip to the one who actually handed you the money. There is no really amount needed for the tip after winning a jackpot. Some players give about a half to one percent of the jackpot. This would be around $5 to $10 dollars if you win a thousand dollar jackpot.
If you notice when you receive the jackpot money there is no smaller bill than a twenty dollar note that is because they are expecting you to give them twenty dollars. It is always handy to have some smaller bills in your pocket like a five or ten dollar bill to be given as a tip. That is more than enough.
It is really your decision and a personal choice to give a tip. It should be based on the quality of service that you receive. You can give a good extra amount if the hand pay jackpot is prompt. Or a little less of the tip if the hand pay jackpot took very long. But if the casino is crowded like during the weekends especially on Saturday nights, expect your hand pay jackpot to take a little longer because there are other players in the casino who are waiting too. So it is good to be considerate and have patience.
Sometime free drinks are offered by the casinos to the players. These drinks are not free for some waiters or waitress in some casinos. Tips are part of their salary and they are required to pay a tax on every drink that they take out on the casino floor. So make sure to take care of the person serving you do not just leave the drink you ordered behind, it will also cause them money.