If there is queue and you are at the end of it, I think you could safely assume that there is nobody in front of you who is disabled - who knows how soon someone who is might join the queue after you. .... However, my point is as somebody else mentioned, not all disabilities are visible & nobody has the right to comment on somebody else using those facilities unless they are certain that person is not disabled.
I had both knees replaced (at the same time) and for quite a while could not use normal toilets as they were too low & couldn't bend to sit. Was I truly disabled - no, but I did have to use the toilet, so for a time I used these toilets when I was out. I was only once commented upon when leaving the toilet that I wasn't disabled, I just looked at her and said 'How do you know?' it amused me greatly to see the embarrassment on her face. Never judge a book by its cover.
I would hasten to add that I've never had to use one since, but if I was desperate, I would. I'm sure fellow queuers would rather I did than left a puddle on the floor!
I think there is a big difference in using a disabled toilet to parking in a disabled/or mother & baby bay without a blue badge or a child. That is just selfish and lazy.