Fuel Pump Or Relay 1980 450SL Died

[xkeots]

Hi All,
I was driving my 80 450SL today all over and when I was about to go home it wouldn't start. I tried cranking it, pushing the plate in the intake down (It wanted to start) I couldn't hear if gas was going in when I pushed the plate as there was a lot of traffic on the street.
I was able to get a cup of gas and pour it into the F.I intake and it started for a moment.
I had to have the car towed.
Would you all agree with me that it is either the relay or fuel pump?
What is my best way to check this and also after 27 years of this car being on the road should I just change them both?
Thanks
Jack
1980 450 SL