If you find Time Machine’s taciturn silence on what it has and will do a bit frustrating at times, the command line utility tmutil can offer you a bit more insight.
In particular, if you’d like to see what Time Machine is going to back up just before it’s about to run, execute the following command in the Terminal:
That will compare the machine as it is now against the last backup (See the man page for more options on the output). It took about 5 minutes on my mac, but I have a small hard drive. It could take somewhat longer depending on how much data you’ve got to trawl through. Remember that you’re doing a diff on a moving target, so the less you can use the machine for other tasks while this command is running the faster it’ll complete.
On the other hand, you might want to know more about what Time Machine has just done after a backup. Which files did, or did not, get changed, added or removed?
You can find out what got backed up after the task finishes by comparing the most recent snapshot (the one that’s just completed), with the previous one (this is much faster, took about 35 seconds here). You can do that in Terminal, too, but the syntax can be a bit tricky. To make it easier, here’s a handy AppleScript that will do it for you. Just pop it into the (Apple)Script Editor.app and hit the ‘Run’ ▶︎ button.
# This script compares the most recent backup to the previous one
# indicating what was changed, added or removed
set backupList to paragraphs of (do shell script "tmutil listbackups")
if (count of backupList) < 2 then
display dialog "Sorry, couldn't find anything to compare. Try again after the next backup"
set path1 to quoted form of item -2 of backupList
set path2 to quoted form of item -1 of backupList
do shell script "tmutil compare " & path1 & " " & path2 & " | open -f"