A library catalog record usually indicates the number of pages in the Format line in the item the Details. It will be written in the format 123 p. (actual number will vary).
For example, this catalog record for Middlemarch by George Eliot shows this edition is 795 pages long.
You could also search for a book on Amazon.com. Page numbers can usually be found in the Product Details section of an item listing.
Be sure to check the edition! The number of pages can vary from one edition of a book to the next.