Wordpress - How do test if a post is a custom post type?

if ( is_singular( 'book' ) ) {
    // conditional content/code
}

The above is true when viewing a post of the custom post type: book.

if ( is_singular( array( 'newspaper', 'book' ) ) ) {
    //  conditional content/code
}

The above is true when viewing a post of the custom post types: newspaper or book.

These and more conditional tags can be viewed here.

Here you are: get_post_type() and then if ( 'book' == get_post_type() ) ... as per Conditional Tags > A Post Type in Codex.

Add this to your functions.php, and you can have the functionality, inside or outside of the loop:

function is_post_type($type){
    global $wp_query;
    if($type == get_post_type($wp_query->post->ID)) 
        return true;
    return false;
}

So you can now use the following:

if (is_single() && is_post_type('post_type')){
    // Work magic
}