fuel pump problem

[jude]

my 1989 300e has, over the past 6 mos. or so, refused to start a few times, usually after i have turned it off for a brief period of time-- maybe 5 to 10 min. several weeks ago it would not start at all and my good friend who plays around with old mb diesels said he thought it was a fuel pump problem. the car cranks fine, it will start with ether, and he can hear the relay click when he turns it on. he doesn't know though whether it is the fuel pump or the relay that is broken. he thought he would just replace the relay to see if that was the problem. he stopped short on that fix when we found out the relay cost over $100.
if there anyway to determine if it is the relay or the fuel pump?
any help would surely be appreciated. thank you,
jude