If Statement Post Formats: No post format selected

use get_post_format:

$format = get_post_format();
if ( false === $format )
    echo "no post format";

EDIT – an example using a switch statement:

$format = get_post_format();
switch( $format ){
    case "aside":
        echo "aside";
        break;
    case "chat":
        echo "chat";
        break;
    default:
        echo "standard";
}