[R] possibility to determine whether a script is "sourced"?

[Jannis]

Dear R community,


I am running some rather large scripts either by executing single blocks of them separately or by executing them at once via 

source('script.R')

Now there is a lot of code in those scripts that I only want to run when I run them manually, so I wonder whether there is any way to automatically find out whether the script is run manually stepwise or whether it is run via source. Is there such a possibility? 

Thanks a lot for your help!
Jannis