Yes, use the statistics collector. The manual for Postgres 9.5:
28.2. The Statistics Collector
PostgreSQL's statistics collector is a subsystem that supports
collection and reporting of information about server activity. [...]
It can also count calls to user-defined functions and the total time
spent in each one.
track_functions enables tracking of usage of
Bold emphasis mine.
Enables tracking of function call counts and time used. Specify
pl to track only procedural-language functions,
all to also track SQL and
C language functions. The default is
none, which disables function
statistics tracking. Only superusers can change this setting.
Note: SQL-language functions that are simple enough to be "inlined" into the calling query will not be tracked, regardless of this setting.
So this is off by default. If all relevant functions are
LANGUAGE plpgsql, it's enough to set it to
postgresql.conf and reload.
track_functions = pl
Postgres starts to gather information. After an adequate period of time, check the track records. Careful though, some important functions may not be called in weeks. Your call ...
How to access gathered statistics? The manual again:
When statistics collection is enabled, the data that is produced can
be accessed via the
pg_statio family of system views.
Refer to Chapter 27 for more information.
SELECT * FROM pg_stat_user_functions;
Or you can easily look it up with pgAdmin (or other GUIs?). In pgAdmin (3 or 4), select the function and look at the "Statistics" tab top right: "Number of Calls".
There are no true "stored procedures" in Postgres, just functions. The term "procedure" is used synonymously.