Unfortunately you cannot visually sex rainbow lorikeets. Both male & female are identical. You can take the bird to an avian vet & get it DNA tested. This wil determine the sex of your bird.
(NSW avians) Dr J Gill BVSc MVM MACVSc, Canley Heights Vet Clinic, Shop 6, Harden St & Avoca rd, Canley Heights, NSW, 02 9604 9792