Unfortunately most manufacturers don't publish this information and it's left to us to find apropriate eyepiece/telescope combinations through trial and error. Pentax is an exception:
http://pentaxplus.jp/archives/tech/xo-xw/64.html
If I'm reading the diagram correctly, the solid and dotted lines represent two focal surfaces formed by the eyepiece (tangential & saggital) with the point of best focus lying between the two. The farther this point of best focus lies away from the vertical axis, the greater the field curvature. You can see that the 14, 20, 30 and 40mm XW's have a field curved in the opposite direction to the other focal lengths. When the solid and dotted lines are close together (as in all of these eyepieces) astigmatism is minimised.