How can I open the file when I type < (file name) in terminal?

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I'm wondering if there is a way to tell if a redirection is present in the shell command?

I wonder if there is a way to tell the shell command about redirection

No. From your program's point of view there is no difference between these two cases:

./Myprogram < input.txt

./Myprogram

In both the cases the program is not taking any command line argument and would get it's input from the standard input. In the first case it's the shell that is connecting the contents of the file input.txt to the stdin of your program your program knows nothing about this.

 is missing. from a program's point of view 
There is no difference between the two cases below.
Two examples:

./Myprogram < input.txt
./Myprogram

In both cases, the program does not acquire command line arguments
Redirection receives input from standard input.
In the first case, the contents of the input.txt file are: 
The shell that connects to the program's stdin. 
The program knows nothing about this fact.