I always have difficulty between Euphorbia lathyrus and Euphorbia oblongata. The identification key in Vicflora separates them thus. Upper stems hairy (oblongata); upper stems hairless (lathyrus). Were the upper stems hairy or hairless?
I go by the symmetrical arrangement of the leaves on the stem and the shape and coloration of the leaves. Euphorbia oblongata is untidier and the leaves more ovate and paler. Another test that can be done on weeds is E. lathyrus is biennial/annula and single stemmed so it can be pulled out more easily than E. oblongata which is multistemmed perennial.