Tradition for many years has made the cathode of diodes the feature to marked with a band or other indicator on the body of the part. LEDs have messed with tradition a bit partly because they are not always thought of as diodes any more but rather as a part with a more positive or plus (anode) contact and a more negative or minus (cathode) connection, like a polarized cap.
And as you know, some caps mark the plus (+++++) like many tantalums and some mark the minus (-----) like many aluminum caps. Somehow we get caps right most of the time.
Whether there is a standard for LEDs or not, if the LED manufactures do not follow it, it leaves it up to us (the contract manufacture) to get it right. I agree, LEDs are harder to know for sure, without opening the spec sheets and in some cases the schematics as well.
It is up to the design engineer to make it clear, based on the LED's spec sheet, how to install the LED on the board based on some visual feature. If there is a question, it is up to the contract manufacture to get a good answer. Hopefully your customer is open to such communications on your part.
Otherwise, you got a 50/50 chance of getting it right, and if you don't it's usually "your fault." Not the best answer, but I understand your issue.