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 can also find it above an open post:
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.