if [ -z ${var+x} ]; then 
    echo "var is unset"; 
else 
    echo "var is set to '$var'"; 
fi

var is the variable name,

To check for non-null/non-zero string variable, i.e. if set, use
if [ -n "$1" ]

if [ -n "$1" ]; then
    echo "You supplied the first parameter!"
else
    echo "First parameter not supplied."
fi

https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash