A fully qualified physio with the right equipment, a proper physio's bench should be able to diagnose a leg lengh discrepancy, they did for me, rather than a muscle issue which feels just like a leg length discrepancy, but with help does clear up.
My physio said to me you have a lower length below knee discrepancy and a femur or upper length discrepancy of totalling 3/4 inch, or thereabouts.
He suggested to me I get a long leg x-ray, where they x-ray you from the floor to your waist, and then measure each bone that makes up your leg, that then clears up what has happened.
The physio was right I have 10mm below my knee, and 12mm above my knee, surgeon denied I had a problem, but I did, could not walk, the more I walked the more pain it caused me, until I worked out that a shoe insert, helped big time, even though the surgeon did not want me to use shoe insert, he said I would get used to the leg length difference with time.
Problem is I have spinal stenosis, ruputered disc S1-L5, bulging disc L5-L4, and lessor bulging disc L4-L3. I believe the disc problems were caused by my trying to walk without the shoe lift, as I cannot prove I had no spinal problems prior to my surgery, no pain, no discomfort, other than spinal stenosis, but we all have that in our 60's to some extent or other.
Shoe life and physio in the local heated swimming pool, non-weight bearing, 3 times a week for 3/4 hour at a time, had to build up to that time, finally solved the problem with pain, took a very long time, as my inabitily to walk only weakened more already weak muscles caused by the surgery.
2 years on I am back to normal but for the shoe lift, or should I saw heel lift, and or local shoe makers adjustment to my shoes, 12mm only, as I don't have to adjust for what is normal for me the 10mm below my knee, I grew with that, and had it my entire life.
Very frustrating that I had to do most of the working out of my problems myself, physio some help, surgeon's team worse than useless.