Mix n match RAM memory in iMac with (potentially) an empty slot remaining
There are multiple iMac models, but in general, look in the apple menu
and select 'About this Mac', then (in the window that opens) click on
the 'Memory' selection.  This will show your memory arrangement
and offer "Memory upgrade instructions" if the computer is internet connected.

If the memory arrangement recognizes all the memory you have installed, it's
working (though matched pairs are often preferred).