Blue rams are easier than bolivans ... the females have the pinkish tummy and male's dorsal rays on 2 and 3 are extended ... for sexing bolivians, see the BRC thread ... great pics there for that species, which isn't as easy to tell as the blue rams.
Shorter/plumper/pinky-purple belly, shorter pelvic and dorsal fins, iridescence inside the black spot(not always an indicator but pretty common) = Female.
Higher bodied, slightly thinner, generally longer pelvic and dorsal with 2nd-4th rays (usually only two) elongated, lack of pink-purple belly and usually lack/less iridescence inside the black spot (on the flank of the fish) = Male.
Female. You can see that even she has an extended dorsal ray:
Showing the iridescent spots inside her black spot:
Typical purple belly - breeding dress:
Two different physiques:
This male shows the higher, flatter body physique that I talk of: