how much does it cost to replace the fuel pump and how do you know when the fuel pump is the problem

Depending on where the pump is located the labor could be pretty expensive. The pump should be checked for proper voltage and ground and that there are no blockages, then if it still doesn't make the pressure specified by the manual it is "not working". Checking fuel pressure alone can not tell you specificaly that the pump is "bad", but it can tell you it is most likely still "good".

One way to tell is to take a friend with you and go out on the highway. Open the car up pretty much and if it seems to hesitate or no power at all at top end the pump is weak and isnt delivering the right amount of fuel to the motor. I had a '91 Spirit do that to me and i found out that way. it may be different on yours im not sure. i dont deal with the forgein cars.