dead giveaway
{n.} (stress on "give")
1. An open secret.
By mid-afternoon, it was a dead giveaway who the new boss would be.

2. A forced or sacrifice sale at which items are sold for much less than their market value.
The Simpson's garage sale was actually a big giveaway.

3. A gift; something one doesn't have to pay for.
The tickets to the concert were a giveaway.

[TahlilGaran] English Idioms Dictionary ▲