How exactly do you know if it's "Worth" it? (Question's)

[Shannon X.]

I'm taking in terms of rehabbing.. how do you generally figure out if the house is worth buying and know that you will make a good profit? if paying in cash how much should you bring down the price, but still beat all the other offers? if the house is foreclosure and most of the work has been done, what could you do to make the value of the house go up and resell it for more?