One of the new items in DW8 is the ability to add a file compare utility that will let you examine differences between files you are working on and remote files.
If you take a look at the preferences menu in DW8, there is a new entry called "File Compare" that lets you point DW to your diff program. Below that is a link to a "help page" to learn more about what you would need to do to get this feature working.
Unfortunately, the link takes you to a DW8 partners page with another link to a company called Araxis that wants to sell you a diff program for $129US.
For some reason, I really hate this sort of thing. It feels the same as if you just went through the process of buying a new car and when you go to the parking lot to drive it away, the salesman pipes up and says, "Oh yeah, the tires are extra". Crap.
I guess the marketing department has to get their fingers into the project somehow. I'm sure Araxis cut some great deal with some relative that works at Macromedia.
If you would still like to try out the compare features in DW8, there are several free, and in some cases, open source, programs that will do the job. I like the WinMerge product right now. It's free and open source. There are several others out there that charge a fee. Some are quite inexpensive. Plus, you could always just use windiff on windows.
To use it once you have it installed, you simply right click on a file in the files panel and select "compare with remote" to have both files opened in the program so you can examine or reconcile any differences. If you use control-click and select two local files, they can be compared as well.
In addition to the DW8 integration, WinMerge does a nice job of allowing you to compare entire directory trees to see what files need your attention. This comes in really handy if you just want to check on DW to make sure your site setup is working as expected.