There are several ways to determine that. This is one: Open Task Monitor | Performance tab | select Memory. It will tell you how many slots and how many in use, and total memory size. e.g. Slots used: 2 of 2
Without taking the rear off, how do I know if my system has 1x8GB or 2x4GB Ram please? Do Dell normally use both slots when fitting memory or do they like to leave a slot free for upgrading? Thanks, Colin   
G3 15 3500 memory configuration.