First run it with all the existing/installed RAM modules. If it comes with no error, all is good.
But if it starts showing errors, Stop testing. Errors/red lines means one or more RAM is faulty. But
the fault may occur due to a faulty DIMM slot, too, which is a motherboard component. Using memtest86+, you can discriminate between a faulty RAM and a faulty motherboard.
How? Say you have two RAM sticks and two DIMM slots. You obtained errors at the test with all RAM sticks installed. Now,
remove all the sticks but one. Test it in all the available slots, one by one. Continue the same procedure for all the available sticks.
How to make the inference that is it a RAM issue or it is a motherboard issue? Suppose you have got the result like that:
test|Slot1|Slot2
RAM1|
Error
|
Error
RAM2|Good|Good
It is a RAM, a bad RAM.
But if you have got a result like that:
test|Slot1|Slot2
RAM1|
Error
|Good
RAM2|
Error
|Good
It is a motherboard issue. The particular slot is bad.