How do I find out if there are still free RAM slots without opening the computer?
You can find this information in the Windows Task-Manager. Start the Task-Manager with the key combination CTRL + Shift + ESC. The first time you open the Task-Manager, it will offer the More details button at the bottom.
Open the detailed view and select the Performance tab. Then click on Memory on the left. At the bottom right you will find the number of available and free slots.
The information is usually reliable. However, this is not the case if soldered RAM is installed, as can be the case in notebooks and mini PCs.