A hint for less experienced players - if all the numbers in all rows can be flipped and read exactly the same (e.g. 2,3,5,3,2) then the puzzle will be symmetrical horizontally (i.e. if there's an uneven set of numbers as in my example - the 5 will be centred). It's the same for columns - vertical symmetry. If all columns and all rows can be flipped then the whole puzzle is perfectly symmetrical. Chano for one has done loads of beautiful perfectly symmetrical tiles if you want to practice. Hope that helps!
A hint for less experienced designers--that's not always true. there are plenty of ways to make a puzzle that appears symmetrical but is solved asymmetrically.
Hope that helps!