I generally check diff fluid level with my finger -the transmission fluid could be at any level below the hole -especially if the trans is warm. I've jammed my pinky in there, just to see if I could feel fluid. This is not the way to do it, but I tried it anyways. The correct way to check the level on the transmission is to open it up and add fluid until it runs out.
I don't see your average mechanic at Jiffy Lube just "throwing fluid away" to see if the transmission is full or not. They'd open up a hot tranny, and since no fluid came out, they would assume that it was full. After all, the truck is running.
I have a cool pyrometer at work -it's like a little temp sensing gun. It's come in handy for about a dozen jobs around the truck. I'd like to get one, they run about $150. I'll be that it would do a fine job of checking the temperature of the transmission pan.