One of the fabulous things about visiting e-book/e-manga stores is that you can almost always read the first 5-20 pages. Less for manga, more for text. Generally, this access will be labeled as 試し読み (tameshiyomi). Sometimes it is called 立読み(tachiyomi). Click on the link and a new window will open up with the preview.
Sometimes you will find books/manga that are advertised as being temporarily fully available as tameshiyomi. Look for this kind of note: 期間限定 試し読み増量版 and make note of the ending date of the offer. Here is an example from BookLive.