Look at the genre. Romance novels sometimes can have some steamy scenes, but if you avoid the ones that are known for being more 'adult - oriented' within the genre, you'll be safe. Also, books that are targeted at a young adult or children's audience are less likely to have such content. You can also ask librarians for recommendations. They are trained to know what books are appropriate for different audiences.
One way is to check reviews. If a book has reviews that mention inappropriate content, it's probably best to avoid it. For example, on Goodreads, people often flag books with such content.
Read the book's description carefully. If it hints at any improper relationships or adult - only themes, it might not be for you. Also, some publishers have a reputation for publishing more family - friendly or 'clean' books. For example, some Christian publishers tend to avoid such content. Another tip is to follow book bloggers who focus on 'clean reads' or books that are suitable for a wide range of audiences.