How to check if a file is binary?

Use utility file, sample usage:

 $ file /bin/bash
 /bin/bash: Mach-O universal binary with 2 architectures
 /bin/bash (for architecture x86_64):   Mach-O 64-bit executable x86_64
 /bin/bash (for architecture i386): Mach-O executable i386

 $ file /etc/passwd
 /etc/passwd: ASCII English text

 $ file code.c
 code.c: ASCII c program text

file manual page