Counting rays isn't hard - you just have to know where to look.
Different tail types also have different amounts of rays. VT's, for example, usually have two, as do CT's (though they can sometimes have more - up to 4 or 8 sometimes). HM's can have up to 8, but usually can have 4 or 6. PK's generally have two (traditional PK's) and HMPK can again have 4-8.
I drew a little diagram-thingy for you. This betta was taken off aquabid for learning purposes only.
Basically, when you look for rays, they always start off at the base of the fin as one big ray, then branch off about 1/4 to halfway down the main ray. It's the branching you want to count. It can be tricky to see sometimes, but if you look closely on a lot of pictures (go to aquabid and practise this - most pictures are usable!) you can see the ray branchings.
Here's the sample I have for you - hope it helps!
