Unless you hear someone saying, "Wow, we're scamming the system!", then you have no way of knowing that someone is doing so just by looking at them.
There are probably 100 different medical reasons that someone could look "normal" yet have a legitimate clinical reason why they can't stand for long periods of time or walk long distances. Off the top of my head, there are all types of back injuries, hip injuries and knee/ankle injuries that don't stop someone from walking a mile or so normally, but that over the course of a long day would leave the person unable to walk anymore without intense pain.
And then there's the stuff like severe asthma, and stamina and endurance from a stroke or trauamtic brain injury, and the list can go on and on.
In any of these, you wouldn't be able to look at someone and know their legitimate medical issue. You wouldn't even know it if you see them spring up from the scooter and walk into the normal line without a limp or without any other obvious difficulty ... think of, for example, someone with a bad back injury that if just fine until their muscles fatigue (like from walking a few miles)and then they can't even stand up from that point on, or someone with a bad heart condition who is restricted from doing all the necessary walking but is well enough to ride non-thrill rides.
There are some ride/attraction queues that are totally wheelchair accessible, and in such cases, NO ONE, whether in a wheelchair or an ECV, should be permitted front-of-the-line status. With a ride such as Spalsh Mountain, it's obvious that someone in a wheelchair or ECV can't walk through that queue, so of course they need some sort of alternate arrangement. But the queue for Soarin' for example, seems totally accessible for any wheeled conveyance, and it's truly unfair that someone in a wheelchair or ECV would be granted front-of-the-line status ... the independence movement is about removing barriers and it is not about putting people up on a pedestal and bending over backwards for them "just because."
Anyway, they're called "invisible disabilities" ... yes, there can be some scammers out there. But
wouldn't you rather let a few scummy people scam the system instead of stopping legitimately needy people from enjoying the magic of Disney? 
