Technically speaking, all gambling winnings are taxable. However, not all gambling winnings are legally required to be reported to the IRS. For example, you could walk into a casino, put $20 into a slot machine, win $100, and walk out. The IRS would have no idea and you'd have an $80 tax-free profit.
On the other hand, there are some formal thresholds above which there will be a paper trail. The requirements vary depending on the type of gambling, but if you have a single win that exceeds a certain amount of money, the gaming establishment will collect certain information from you (such as your Social Security number and a copy of your ID) and will issue you a Form W-2G to document the win. And keep in mind, if you receive a tax document in the mail, that usually means the IRS received a copy as well and knows about it.
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A quick guide to the various tax documentation requirements
Horse racing or general gaming: If you win $600 or more on a single event, or over 300 times the wager amount, whichever is greater, it is subject to IRS reporting. For example, if you wager $20 on an event and win $800, this would be 40 times the wager amount, which would not trigger a tax form.
Slot machines or bingo: Wins of $1,200 or more are considered "hand pay" jackpots (meaning they are physically paid by an attendant, not by the machine itself), and tax information will be collected. However, this only applies to individual wins. For example, if you win $1,000 on one pull of a slot machine and $400 on the next pull, you can simply cash out the $1,400 even though it exceeds the threshold.
Keno: Wins of $1,500 or more at Keno will trigger the W-2G requirement.
Poker: If you win $5,000 or more at a poker tournament, a W-2G is required.
Table games: In most cases, casinos generally don't have to issue W-2G forms for table games, regardless of the amount won. Technically, the same "$600 and 300 times the wager" threshold that applies to horse racing applies here, but it's rare for a single table game win to exceed 300 times a wagered amount. If you win a side bet or progressive jackpot, it could apply.
If the casino takes your information for a win, you can expect to receive a W-2G for the year in which the win took place, and it should arrive around the same time as the rest of your mailed tax documents (usually mid-to-late January).
It's also worth noting that these rules apply to non-cash prizes as well. For example, if you play a slot machine and win a car, the same tax reporting threshold applies as if the prize was paid in cash.