declare can be used to determine if a variable or array has been defined. In this example, declare is used to determine if the foo variable has been defined.

declare -p foo

If the foo variable has not been defined, something like this should be returned.

bash: declare: foo: not found

If the foo variable has been defined, something like this should be returned. Two dashes (--) suggests that foo is a variable.

declare -- foo="bar"

If the foo array has been defined, something like this should be returned. -a means that foo is an array.

declare -a foo='([0]="bar")'