I've used System Information for Windows (SIW) for years and found it a fantastic toolkit. Not only does it show number of memory slots, it shows what's in them, the maximum capacity, the manufacturer, part number, voltage etc etc
That's just the tip of the iceberg - you can find out software licence keys (Windows, Office and others), certain passwords, CPU information, laptop battery wear, hard drive temperatures, the list is endless! (I've used it for years and must have used a fraction of it's features.)
Not only that - there's no install, it's just a 2MB executable so ideal for putting on a USB drive for PC diagnostics.
Unfortunately, the free home version has been discontinued, and even if you can find it it doesn't work on Windows 10. I guess Gabriel Topala finally wanted more than the "Buy me a coffee" option, but to be fair he's certainly earned it!