The SWI-Prolog documentation pages for built-in predicates have a nice feature to show the Prolog source code for a predicate by clicking on 
.
The feature only appears on the predicate page, e.g.
it does not appear on section pages, e.g.
If you expect the feature for a predicate and don’t see the icon, in the left table of index click on the specific predicate and the icon may be there.
Note: Some predicates are implemented in C and will not have the show source feature.
To identify predicates implemented in C search C files (*.c) for PRED_IMPL, e.g.