Wikipedia does a good job of summarizing the four stages of enlightenment, which are stream-enterer, once-returner, non-returner, and arahant. They are distinguished by their remaining attachments such as clinging to rituals, self-view and doubt to an arahant who is fully released.
There are no objective standards for determining the stage of another individual but there are certainly clues. For example, any expression of self-view is undeniable falsification: Insult their mother and see how they react.
However, the stages along the path are subjectively but entirely testable and in minute granularity. All schools of Buddhism recognise the authority of the nikayas/agamas, in which the Buddha describes the dhamma and practice in detail. While many factors are enumerated, in whole the path and attainments are not linear. For example the four material jhanas are sequential but the eight-fold path is better described as cyclical such that the development of each characteristic reinforces the other characteristics of the path. Wisdom, virtue, and concentration are developed in parallel.
Some traditions emphasise an instantaneous enlightenment (chan, zen), some hold a view of a prior enlightened Bodhi nature that must be revealed, while traditionally it was presented as a methodical process. But I don't think it would correct to say that different traditions have different notions of enlightenment. I hesitate here on certain teachers, particularly in Tibetan/bon traditions, who assert the rebirth of arahants. The rebirth of enlightened worthy ones contradicts the Buddha's own definition of nibbana/nirvana, which literally means unbinding, fading out, the end of rebirth, not entering any womb. But rebirth is difficult to objectively test without some fancy super-mundane instruments.
Namo tassa bhagavato arahato samma-sambuddhassa
In the name of Blessed One, the Exalted One (arahant), the fully-(self)-Enlightened One
Among the enlightened, there is a semantic distinction between buddhas, arahants, with residue and without. A buddha is self-realised while an arahant followed the path of a buddha. An enlightened one with residue is still supported by the khandas (he has a body and is alive) while after death (paranibbana) an enlightened one is not supported by the khandas, not supported by a body, is dead and will not return to any womb.
Perhaps a more useful question, if I may: "How does one choose a worthy teacher?"