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To check a multi-lines variable with spaces, try this:

[[ $var = *[$" \t\n"]* ]] 

Or with xargs:

[[ -z $(echo $var | xargs) ]] 

Or with sed:

[[ -z $(sed "/^$/d" <<< $var) ]] 

To check if variable has only spaces, including in a multi-line variable, try this:

[[ $var = *[$" \t\n"]* ]] 

Or with xargs:

[[ -z $(echo $var | xargs) ]] 

Or with sed:

[[ -z $(sed "/^$/d" <<< $var) ]] 

For multi-lines variable with spaces, try this:

[[ $var = *[$' \t\n']* ]] 

Or with sed:

[[ -z $(sed '/^$/d' <<< $var) ]] 

To check a multi-lines variable with spaces, try this:

[[ $var = *[$" \t\n"]* ]] 

Or with xargs:

[[ -z $(echo $var | xargs) ]] 

Or with sed:

[[ -z $(sed "/^$/d" <<< $var) ]] 

With regex there's an ultimate way to check for spaces:

[[ $var =~ \s* ]] 

Unlike other approaches here, it also includes multi-lines variable with spaces.

For multi-lines variable with spaces, try this:

[[ $var = *[$' \t\n']* ]] 

Or with sed:

[[ -z $(sed '/^$/d' <<< $var) ]]