I know you can change the oil pump from under the car on a 540 and I would think it would be the same for you. As for signs of a bad oil pump I would drain the oil and open it up and have a physical look at everything. I would think the next thing would be to check oil pressure. I'm not all to sure how to go about that but i'm pretty sure somebody here can tell us how its done. I dont know if I thought there was an issue with my oil pump I think I would change it out, has to be much cheaper than an engine rebuild.
Johan