Slidewise will warn you that there are Excel files embedded behind your Charts, however, it will not tell you if there is anything extra or sensitive in them - that you have to find out manually.
The first thing to look out for is the size - if they are low KB, then there is less chance of lots of extra data being in the files. These files for example probably only contain the data for the chart.
However, if you have a file that is multiple MB then that is definitely much more data than would ever be needed to drive a single chart. These files are much larger and contain multiple tabs of data that isn't visible directly in the presentation but can be accessed by anyone with the file.
The similarities in file size above is due to the same Excel file being embedded multiple times, once per chart.
You can check the files easily using the option in Slidewise. Just right-click on the Excel node and select Edit Data in Excel. This will open the Excel file behind the chart on the tab that the chart comes from so that you can investigate.
We recommend that you look at what is on the other tabs to decide if it is a problem to share. If there is something problematic, then you have a few options.
You can remove any tabs or cells that aren't needed for the chart, although you'll need to make sure your chart isn't referencing data on the other tabs. Then just close the Excel file, which will prompt it to update in PowerPoint.
Make sure you check for hidden tabs, in case the data you want to avoid sharing is on a hidden tab. Simply right-click on any tab in the Excel file and if the Unhide slide option is available then click on it to make them visible for checking.
Alternatively, inside Slidewise you can use the Remove Data option to strip the Excel file away leaving just the chart. You can still format it like a chart, but you will no longer be able to edit the data. You have the option to do this on individual Excel files, or for all of them at once.
A note on file size
If you mostly want to reduce the size of the Excel files embedded within your presentation, then the option above of removing data using Slidewise is one approach.
However, if it isn't working then you may need to extract the Excel file and do some more fine grained investigation. Click the Edit Data in Excel option and then Save As... from Excel. We recommend following the steps outlined in our guide to why Excel files get so big and what you can do about it.
Once you've fixed the file size, you can replace the chart in your presentation from your now smaller Excel file and delete the bloated version.