Yup, the main slots are the white ones, the secondary slots are the blue ones.
Hence you should put your RAM sticks (possibly same manufacturer, model and size) in the first white slot (near the cpu), then in the second white slot, then in the third white slot.
You should put your other ram sticks, in the same order: in the first blue slot (near the cpu), then in the second blue slot, then in the third blue slot.
That's why the fifth slot should be empty in your case.
When I suggested the first configuration, I thought it should be better to put first the 8GB modules (in the white slots) because probably they get priority over the other slots.
But you have got only two 8GB modules, therefore I thought that probably it's better to leave the sixth empty and to fill the other blue slots with the other three 2GB modules.
Now to your problem (how can I test the sticks?), I tested only once a 2GB memory I bought with hcidesign memtest (that runs under windows) and other times sporadically with Windows Memory Diagnostics, thus I'm not the most experienced user about how to test the ram.
Nonetheless, I can read on
memtest86+ (memtest.org) forum, on canardpc.com (click):
one successful pass of memtest will give you a pretty good idea that your memory is ok, only in rare cases will there be errors showing after the first pass. To be sure though simply have the test run overnight or even for a couple of days depending on the level of importance of the system.
Passmark Memtest86 says:
When MemTest86 boots, a splashscreen is displayed with a 10 second countdown timer which when expires, automatically starts the memory tests with default settings.
Currently, the most updated software is memtest86: V7.1 --- 05 Aug 2016
Hcidesign memtest: 4.6 --- 29 May 2016
Windows memory diagnostic tool: it could have been updated when windows 10 came out, or it could be the same version present in windows vista... Who knows? If it was updated for windows 10, it could be near the previous tool.
memtest86+: 5.01 --- 27 Sep 2013
Imo, you should wait to run the (long) ram test and use your computer to see if you still get the same errors you experienced before the change you made some hours ago.
If they disappeared, you can simply run the windows memory diagnostic tool with its default settings, or you can change the number of passes also with it, just for the curiosity to verify your ram sticks.