Finding the Contact ID on the Overview tag of a Contact record is easy; it's clearly shown smack-dab in the middle of the profile. But when switching to other tabs, like Giving or Notes, the Contact ID is no longer displayed on-screen. Finding the ID requires switching back to the Overview tab.
But you can use this handy tip to find the ID no matter what tab you are viewing!
Just look at the address bar in your web browser. There will be a string of numbers at the end of the page url; those numbers are the Contact ID.
No more tab switching required!