This is a complex issue.
The opposites can be true too. I have seen Gurus/disciples who put on a great show of humility in their speech, but on probing, they are quite different. They are proud about their knowledge, how their God and their philosophy is superior/more complete than others. They also resent anyone questioning or disagreeing with their views. But on the surface, they are very humble. Interestingly, they believe they are being humble.
On the Bentley theory, we could say, the true Guru is one who sees no difference between a Bentley and a Kia. Both are the same to him and he rides a Bentley only because that is what was given to him by his disciples.
On collecting fat donations, most Gurus - once they attain some level of popularity - create welfare organizations (building temples, Ashrams, Meditation Centers, Hospitals, Schools, etc). This is justified by the idea that the Guru now has celebrity status and can leverage it to raise funds to help the less privileged and to spread divine knowledge and so, there is no nothing wrong in doing it. They call it "service".
In short, there are no clear black and white rules in identifying Gurus. It is purely subjective. Some will choose the person who has given up society and lives a frugal life (loin-cloth, alms..). Others favor Gurus who are famous, with multiple ashrams and collections of Rolls Royce cars.