How can you tell if an older book is a first edition?

On the page with the copyright and publishers info and whatnot, it will have the copyright date and the date it was published. It will also say if it was previously printed, and if so, when and where.

Sometimes it takes a lot of work to make that determination. You can't always trust what is printed in the book itself. In the case of Huckleberry Finn, for example, you need to go to a bibliographical source to look for "points" and then see which of those points are actually in the book in your hand. Often, the older the book, the harder it is to tell. Good luck.

no copyrights. some books it says if its first edition also.

There are no previous copyright dates listed.