A good way is to look at the synopsis of each book. If one book seems to be building on the events or characters introduced in another, it's likely that the one with the earlier events comes first in the order. Also, reading reviews from other readers who have read the whole series can give you hints about the correct order. Some reviewers might mention if a particular book should be read before or after another one in the series.
Check the series number on the book cover. Usually, it's clearly marked if it's part of a series and what number it is.
You can start by finding out which book was published first. This can be done by searching for the books on a bookstore website and looking at the publication dates. Then, look for any information in the books themselves. Sometimes, there are references to previous events or characters from earlier books in the series. If you're still not sure, join an online book club dedicated to Cassie Dewell novels. Members there are likely to have discussions about the correct order and can help you out.