To see a post's ID, find the post in your Q&A feed and hover your mouse over the triangle so that the dropdown menu appears. You will see the post's ID at the top of the menu:
To link to another post, simply type @ and the post's number. For example, if you'd like to link to post 47, type @47 in the body of your post. This way, users will be able to view post 47 by clicking @47 in your post.