WordPress: Check if it’s a Post or a Page

When inside the loop, a simple if statement should suffice:

if (get_post_type() === 'post') {
    //post
}

if (get_post_type() === 'page') {
    //page
}

Outside of the loop, you might need to acquire the page/post id from the global post object first, and pass that to the get_post_type() functions:

$id = get_the_id(); //current post/page id
if (get_post_type($id) === 'post') {
    //post
}

if (get_post_type($id) === 'page') {
    //page
}

Similarly, you can use the is_page() and is_single() functions to the same end. This is possibly the better choice, since it’s a simple true or false check rather than a check against a string value:

$id = get_the_id(); //current post/page id
if (is_page($id) ){
	//page
}
if (is_single($id) ){
	//post
}