It has to do with the chips used for the ram, if your ram doesn't have a heatsink on it you'd be able to see the chips on the pcb itself otherwise you'd need to look at it from the bottom to see them underneath the heatsink but if there's only chips on one side of the pcb it's single rank otherwise dual rank.

When you look at 8GB modules I'm not sure if anyone actually produces 4Gbit chips anymore which would be what would be needed to make an 8GB stick dual rank instead of the generally standard 8Gbit chips used in them. When you're looking at 16GB modules it's becoming more and more common to have 16Gbit chips used to make them which means they're also being made single rank but there are still quite a few made with 8Gbit chips which are thus dual rank.

Unfortunately none of these things are officially reported by the manufacturers so without emailing them to ask you can't be certain if the sticks you're thinking about ordering are in fact dual or single rank without seeing them first since some sticks that have been known to be dual rank in the past have swapped over to being produced as single rank instead.

Gamer's Nexus did a piece on single rank vs. dual rank with zen 3 when it was discovered shortly after the cpus launched that they behaved differently than what people were used to from other cpus, in some cases the performance gain simply from have 4 ranks between your RAM sticks either in 2x16 or 4x8 format as the current most common setups was near or upwards of 10%.

Of course none of this applies to Intel CPUs but the Intel CPUs also aren't currently the absolute top of the line when considering building something like a dream PC.