Richard Feynman had to consult a mole on his left hand to remember the difference between left and right.

Hold out your left hand and write "left" in the air using it as the L. After a few years of doing this (maybe two), this fixed the problem for me 99.99% of the time.

I actually have a rather big mark on my left hand and my grandmother taught me to differentiate left and right with that mark on my hand, which I still use as I sometimes get confused. Weird but true.

Once I got tattoos on my forearms it made figure out left and right so much easier.

When I was a kid I once wrote L and R on my forearms with a marker both for remembering which was which and to learn their name in english. I think it worked, because I always think of the corresponding forearm when I have to tell left from right.

I have to refer to the hand I hold a mouse in, before that I used to imagine holding an n64 controller.

For most people, the right hand is the one you write with.

For left-handed people, it's a bit more complicated.

It the one I don't write with...

Not so complicated.

my left hand is the one that makes an L shape. Hands flat, plams down, which index finger + thumb looks like an L? That's your left hand.

that has never worked for me. The hand I automatically pick up a pencil with is the right hand. So I make like I'm holding a pencil and then use that hand.