With an extract beer, unless you add more water than needed or boil too much to concentrate the wort more than expected, your OG should be pretty close to the recipe. You don't have to worry about efficiency in terms of conversion or sparging like you would with all grain. As long as the recipe's numbers were good (i.e the person who wrote the recipe put together an accurate one), you should be very close to the stated OG from the recipe.
Even when you know your exact OG, you can't be entirely sure of your FG. There are a range of factors involved in determining where the yeast will take the finished beer.
If the recipe gives a FG or 1.013 to 1.017, and you hit that for 2-3 days in a row, you should be fine. If you're significantly above that range, you might not be ready to bottle.