Rear view ears at speed

In short, you can't.

I find turning my head to the side a bit reduces the noise a bit in one ear, but you can't do this all the time.

A rear view cycle mirror may help you.

There are times you can and times you can't. I've got a regular descent at around 50km/h, which seems to be a bit of a transition. The typical car speed there is 60-70km/h. Cars coming downhill don't make as much engine noise as on the flat, and small new vehicles are inaudible there. It's not all engine noise of course. I've heard a Tesla coming up behind from the wheel noise alone. When you're behind a very noisy vehicle it drowns or other vehicle noises. In fact the last time a car hit me I would have heard it coming had it not been for a very loud train nearby (wing mirror hit my arm as I lifted my hand off the bars to signal. The mirror broke; I didn't). So you can't rely completely on your ears. But they're still very useful (I say this as someone who has and uses a mirror on my main bikes).

Generally speaking you can hear when the closing speed is significant. When you're going slowly you can hear cars at any speed. When you're going reasonably fast you can hear fast cars, but of course you don't get much warning before they catch you up. What's really useful is that you can hear the change of gear and revs before someone pulls out to overtake. At typical urban speeds you can also get quite a lot of information about speed and position of following vehicles.

I once met a chap who had attached long fuzzy fur to the part of his helmet strap that went just in front of his ears. He claimed that it reduced wind noise considerably, same theory as the really fury boom microphones they use in windy conditions. I have no experience, but it seemed to work for him.

I concur with Nathan, don't rely on your hearing for cars, ensure you can see what's coming up from behind and very importantly, make sure you as as visible as humanly possible from all directions.

If you're going fast enough, take the lane.

That way traffic behind you has to stay behind you.

If the corners are the only places where you need to cut in, then take the lane well ahead of the corner - sit in the middle of the lane or in the more-central wheel track. Avoid sitting in the outer-most wheel track, because that might encourage a car to pass dangerously.

Because you're taking the lane at the top of the descent, or well before your corners, then you have time to glance backward and see what's coming.

Your ears loose value on a downhill because of the wind, and the detection range drops. Also, vehicles are getting quieter engines and tyres than historically, and vehicles on the overrun can be very quiet.

Aside, I once had a diesel-electric locomotive sneak up beside me,
on a flat road with a railway to the side.  That was educational.

The speed differential is what matters here - too slow and you'll just clog the traffic. If you can ride at or close-to the posted speed limit then a vehicle should be matching your speed. I routinely do 50/60 km/h descent on a road that is 70km/h at the top and dropping to 50 km/h lower down. Its not unusual to keep up with a car all the way down.

Your estimated 20 mph or 31 km/h might be a bit slow if the traffic is doing 50 km/h or more, so take it all with respect to your personal environment and risk acceptance.