That assumes you know the client where the call or job is spawned from.
The correct tool if you only know the user or want to find the users is SM19 (security audit log).
With a bit of effort you can also extract useful information from the stat-collectors (table MONI and associate files) to get up and running fast enough if the SM19 log was not active before hand.
However Nuno should provide a bit more information about why this is wanted to get a more detailed answer and also show that some search was done before popping the question - as many aspects of this question have been answered before here or have infos in help.sap.com and service.sap.com.
If Nuno does not provide more details or follow-up and nothing interestig turns up then I will delete the whole thread as it just creates noise for others who do search.
Cheers,
Julius