One by itself is very hard to sex unless you have experience with Rosellas. There is no difference in the plumage colouring but the c0ck has a broader beak and bolder head. It is fairly easy to determine when you have some together for comparison.
You can however send a couple of feathers to
DNA Solutions and get an certain answer. Note not to compromise the specimen feathers by touching the shaft.
DNA sexing is by far the safest way. Your Vet can also insert a laparo/endoscope to view the ovaries/testes but this involves anesthetising the bird which is quite dangerous.