Can a nested function determine it is being called within a trigger?

When a trigger calls a function, that function can determine if it is being called from within a trigger by testing for
avalue in the special variable "TG_OP". 

But what if a trigger calls a function that calls a 2nd function via the "PERFORM" command? Can that 2nd function tell
ifit is being run in a trigger? 

Trigger --> Function_A --> Function_B

--Basil Bourque