You are right - on a recumbent you may have to adapt other strategies. But that's fine - the concept still applies. So, lets try this again.

Motorists have one overriding (no pun intended) fear of you, the cyclist: you are going to do something stupid to place yourself in their path.
Yes, I said "fear." They may get angry that you are in their wa,y on "their" road and may, on rare occasions, slow them down. They may question your sanity that you are even out riding on the open, paved roads in the first place. But they fear you may turn in front of them and they wont be able to react in time. Or that the lane is too narrow to allow sufficient clearance for both of you. Either way, they are socially and culturally conditioned to NOT hit you.

The head check is a way to signal to the driver that you are paying attention and won't do anything stupid... in a very real way, it helps soothe their fear of you.
For a moment, lets take this out of the realm of the esoteric and look at it from my perspective.

I ride solo for fitness, mostly out on open country roads. Out in the boonies, cars are fewer, but they are generally moving fast and there is a long reaction time available. Here's where the head check comes in. You may have missed it earlier, but I have painted the SIDES of my helmet in fluorescent colors. When I turn my head, drivers can can see my head turn as a bright color anomaly on the road ahead. It is something like a warning beacon, you might say.

So, motorists first see me as a "blob" in mid lane, because I ride in the half, to one-third lane position. As they get closer, they see my head turn, as I take notice of them first. I hear them coming and I see them in my mirror. Then they see me wave my hand once I've seen them... they may not know why, but they see it. Next, they see me negotiate the lane in their favor where practical, because I saw THEM coming and have the time to make corrections. They know that I am paying attention - in some case, maybe more than THEY are.

I also DO NOT ride with a personal music player/earplugs blasting in my ear. You need all your senses on the road and your hearing is tres important. I use a mirror, too, and check it every 5-10 seconds for approaching traffic. Meanwhile, as I noted, I take the lane where ever practical and work from that most visible of positions. I am always keeping an eye out in front of me, as well. In congested areas, these same rules apply - magnified in importance.

All right, back to the esoteric. The point in all of this is the head check is one part of a greater sense of road awareness, the realization that you must be seen in advance, so others can adapt to your presence. It works well, because I take advantage of the reaction distances involved. But, this mindset can be adopted anywhere you ride. I also drive a car and I see cycling from the motorists perspective. I talk to fellow motorists about cycling, too, and what they think of it. I don't do this to push a personal "cyclists agenda," but to learn what THEIR perspective is.

We can howl all we want our equal rights to the road under traffic laws, but the fact is you are not alone out there - you are sharing the road. If the laws also see us as vehicle operators with rights to share the road, then we have to act like it.

As for the difficult-to-see recumbent, do you use flags, bright colors, strobes and other high visibility devices?

There is nothing difficult to see about recumbents. Not even the really low trikes.

The LWB 'bents tend to be longer than upright bikes. Sure, your head might be a foot lower, but the bike is a foot or two longer which makes up for it.

SWB 'bents tend to be about the same length as upright bikes. Again, your head might be a foot lower, but you're still plenty visible.

The trikes are often significantly shorter -- my head tends to come up to butt level on cyclists on upright bikes -- but the trike is wider and that's pretty noticeable. Many trike users do like flags, however.

Can you see a pothole? Well, any of these bikes are way taller and way bigger than that.

Also, 'bents tend to stick out. Drivers and other riders don't tune them out -- being unusual, people tend to pay closer attention to them. In my experience, this seems to be a bigger factor than anything involving being slightly shorter.

As for looking behind you, if it's not obvious to many -- you are sitting in a seat that tends to make it difficult to twist your torso, and you often can't twist your neck enough by itself to see behind you. So mirrors and signaling your turns becomes even more important -- the mirror to let you see if it's clear, and the signal to replace the visual cue of you looking back.

A more reliable indicator of an action that is about to happen is an arm signal - I make sure to signal before a lane change or turn. A headcheck communicates nothing in particular.

I stick both arms up and wave them around every once in a while, so that motorists will notice me as I ride my invisible bicycle.

I didn't say it was the most effective form of communication, just that it is a form of communication (in addition to telling you what's going on back there.)

It tells people that you might be about to do something.
It tells people that you're aware of the traffic behind and to the side of you.

dahut covered it pretty well.