At 6, kids are still sometimes getting letters backwards. I've seen some kids go from letter-perfect to some reversal.
However - a psychometric assessment (a school counsellor could do it) would show LDs as a wide gap in the sub-scores. As long as you can get access to those sub-scores and the school counsellor is willing to acknowledge these with you, then great. Otherwise you might need to go private.
In the meantime, you could try a few tricks to help her.
A BIG example, from my own childhood - I remember having trouble trying to remember which way round to do lower case B and lower case D. I worked out that the word "bed" LOOKS like a bed, when you write it correctly. The two uprights on each end are the bed posts, and the little rounded bits in the middle are the mattress (or the pillows, depending on which way round you sleep). After that all I had to do was visualise "bed" and I got the letters right.
When my adopted nephew was having coaching to overcome his dyslexia, they used similar tricks. He had to always move his pen in a clockwise fashion, so the letter "g", the first letter in "girl", you begin at the bottom of the round bit. You go round clockwise - that's the girl's head - and then you continue down for the girl's long hair, which is so long she is sitting on it.
There are lots of other tricks, too.
basically, get her assessed if you want it firmly diagnosed or defined. It could be useful to her. But in t he meantime, you've already observed where she's having trouble, so you can begin to help her NOW.
If she's getting her letters reversed ALL THE TIME, then she does know the difference. She just isn't sure which way is correct. That's just a matter of learning what's correct. But if she's getting it reversed about half the time, then you have a BIG problem - she simply can't tell that there IS a difference. That happens in dyslexia. And some teachers miss this because they say, "well, he's getting it right half the time, so he does know..." as my nephew's teacher was saying.
Dyslexia can also be a problem of incomplete brain dominance. Sometimes this is just a slower maturing and it clears up before a diagnosis would be given. Sometimes it's a longer term problem.
Is she showing clear handedness? Left? Right? Or is she still swapping hands? Clear handedness makes dyslexia less likely, but doesn' eliminate it. By now she should be showing a clear preference and if she is not, she could be having trouble with her eyes tracking left to right. When we read, our eyes move in steps from left to right. These are called saccades. These eye movements can be detected with electrodes. In the split instant the eye is moving, it does not register what it sees. it only registers while it is not moving. If her saccades are not developing normally, this can also be a problem. This also can happen in incomplete brain dominance.
I was coaching some dyslexic kids and developed an exercise to try to deal with this - get a small ball, preferably one of those fun ones with a second ball inside floating on fluid - you can get eyeball ones which are the best. The inner ball is weighted and so stays always facing up the same way. But it can work with an ordinary ball too.
You get the child to sit at the table and roll the ball from her left hand to her right. She has to watch the ball roll. The right picks up the ball, passes it back to the left UNDER THE DESK and you repeat the exercise. You are trying to train the eyes to track from left to right in saccades.
Stop the exercise before she gets fed up with it, but the more repetitions the better the brain patterning.
And it's free.
But it's no substitute for an expert assessment. It's just a stopgap exercise.
Good luck in getting some help and some answers.