Look for ones that look to be fed well. No extreme ribs or bones showing under the skin.
Check each of the tails and toes. Make sure none are broken or black. If they are, they might appear black. Black means the area is dead and might fall off.
Now check to make sure they have all their toe nails.
Also, if they are calm enough, touch one of their back legs near their tail, give a little push. This is just to try and get them to get up and run a bit to make sure they are able to get up and walk. When you pick them up, if you hold more the upper body, they should kick their legs to make sure all of them are in your hand. That would be a good sign that the legs are working.
Some pet stores or breeders may feed them to large a food source that may cause impaction.
As per Sexing, check this out: http://www.dachiu.com/sexing.html
It is for bearded Dragons, but they are a similar and I'm sure the photos there would help.
I have no experience with Frilleds, but I assume this would be right since Beardies are similar.
B. Dragon Sexing Photos.