The fuel pump runs all the time that the engine is running and you can hear it if you stand at the rear of the car. A pump in good condition may be almost silent, or produce a little 'sizzle'. As it deteriorates, the noise usually gets louder and harsher. However noisy fuel pumps can often go on for a long time without causing a problem, so you can't be certain,
In theory, a fuel pressure test can be carried out, but that will only tell you that the pump is working at the time you test it.
When I had the problem on my 190 I figured that the cost of replacement ( about $140 for the pump and $40 for the fuel filter) was less than I had already paid for a Bosch technician to fail to diagnose the problem. I was fortunate that this worked for me.