Preview of post

This will depend on if you are using the quick reply box or are using the full reply text box you get when clicking on the reply button in the top right of the quick reply box.
Seen here


Once you have clicked on this reply button it will take you to the "create a post" view giving you a number of additional options to post images format your text and at the bottom of the text box
Here


Are the Preview and BBCode Tabs.
if you change to preview tab it will display your post as it will be seen by others whilst still allowing you to edit and adjust it before you post.

You can also select which tab (preview or BBCode) you would like to have as the default option in your profile setting let me know if you would like help changing that as well

Hope this helps