How to tell < file apart from any other redirection within a bash function?

[Peng Yu]

Hi,

I'd like to tell the first case apart from all other possible cases.
a_bash_func is a bash function.

a_bash_func < a_file

Other cases,

cmd | a_bash_func
a_bash_func < non_file(maybe a fifo or something else other than file)
...

Is the following the best way to perform such a test to know whether
it is the first case within a_bash_func? Thanks.

[[ -f /dev/fd/0 ]]

-- 
Regards,
Peng