Guys
this is all very useful information, but isn't the question really - how do I deal with such variable serves rather than how do I make sure they are delivering a legal serve.
It seems to me that if the person receiving serve was to concentrate on whether it was a legal serve or not when trying to be "ready" for receiving the serve, they are more likely to miss the return of the shuttle!!
In my limited experience, I have found that you need to find your "place" when receiving serve... This is the place where you know you have limited exposure or risk when returning the serve... I tend to stand about 2-3 feet back from the service line, my body weight centrally balanced, off the heels of my feet (on my toes), raquet and non raquet arms upwards and "threateningly" poised & apart for good balance - so as to hope to intimidate the server. FACE the server - your bady angle directly inline with his... Feet apart, but NOT one behind the other as the latter will ruin / minimise your foot movement. Be ready to "pounce"
Practice "springing" from this stance - to the tramlines for a low net serve; to the back of the court to your forehand; to the back.middle of the court for a "round the head" return; moving the racquet and body for receipt of body shot, power serves. It takes some time to get it right - and hey, I've not seen anyone who gets it right all the time..
Feet, stance, and racquet position are key...
hope this helps - it certainly helped me.
Then practice the delivery of serves as the others have so carefully explained - look at the diagrams... Eventually you will gain the confidence and ability to do all of this as second nature... Be patient with yourself - and don't let others phase you (as they so often try to do).. The art of badminton evolves - it doesn't just happen (unfortunately!!)
all the best - and good luck