Somebody posted a picture of it, but look at your city screen, and the icons in the bottom-right-center, where you can turn on emphasize hammers, emphasize food, etc. Above those are two icons - one for hurrying production with slavery, the other for hurrying production with money.
If you have slavery activated, and you have enough population to hurry production, there will be an up-arrow lit up. If you don't have enough population, rolling your cursor over it should tell you how much population you would need.
As for when to do it, that's a strategic decision about whether you want to let your population grow, or whether you really want the item you're building now. Clearly, you should probably use it if your city's population is about to exceed it's happiness or health maximum.