Here are some more tests

True if string is not empty:

[ "$var" ]
[ -n "$var" ]
[[ $var ]]
[[ -n $var ]]
(( ${#var} ))
let ${#var}
test "$var"
test -n "$var"

True if string is empty:

[ ! "$var" ]
[ -z "$var" ]
[[ ! $var ]]
[[ -z $var ]]
(( ! ${#var} ))
let ! ${#var}
test ! "$var"
test -z "$var"

Here are some more tests

True if string is not empty:

[ -n "$var" ]
[[ -n $var ]]
test -n "$var"
[ "$var" ]
[[ $var ]]
(( ${#var} ))
let ${#var}
test "$var"

True if string is empty:

[ -z "$var" ]
[[ -z $var ]]
test -z "$var"
! [ "$var" ]
! [[ $var ]]
! (( ${#var} ))
! let ${#var}
! test "$var"