The block spot on the side of a male blue ram will simply be black. On females, the black spot will have bright blue speckles.
Females also tend to have rose-pink bellies.
These are photos of my pair just before they spawned (about half an hour after the pics were taken) so the females spawning tube in particular is larger than it usually is.
Male
Female
Hopefully you can see the difference.
The first rays of the male's dorsal fin are longer than the females, though sometimes males can have shorter ones and females longer ones.
The males ventral fins (under the head) have extensions, the female's don't.
The top and bottom rays on the male's tail are extended as well, the female's aren't.
Where the anal fin attaches to the body, the body is flat in a male and rounded in a female - though under nourished females also have a flat area, this is quite common in shops.
The fish in shops are usually quite young and males may not yet have developed the extensions to their fins.
Nearly forgot to mention - if the fish in the shop happen to have visible spawning tubes, you can the see the different shapes in the pics.
The female's breeding tube is is the same width all the way down. Looked at from the side, it looks square. It has to be the same all the way down as eggs have to pass through and they are quite big.
The male's is smaller and goes to a point at the bottom. He doesn't have to lay eggs!
If both of yours have cylindrical tubes the same width all the way along, they are both female. At the age they are, they are old enough to tell. As for the fins, fish do vary from individual to individual. Some men look quite feminine, some women look mannish, it's the same with fish. This is why the fins can be misleading.
If you look at the ones in the shop tank and choose the fish with the longest fin extensions, and with an anal fin that overlaps the tail fin you stand the best chance of it being a male.
My 2 males lived quite happily together till I put the females in the tank then war broke out. I had to put one in the QT or they'd have damaged each other. For a few days, the male I left in spent half his time with one female at one side of the tank and half with the other female at the other side. Then he paired up with one and they both attacked the other, so badly I had to put female #2 in the QT as well.
So what I'm getting round to is, if you do manage to find a male keep an eye on them just in case the same thing happens to you.