I have worked with children and families all my working life, and have dealt a lot with these discussions. I did have one family, who, when the 7 year old was being considered for an autism diagnosis, sent the grandfather for assessment, and he got his diagnosis in his 70s!
First of all, there is no definitive 'test'. Assessment usually takes place over time for children - can be more obvious in adults. Some children mature out of certain traits - in others they become more embedded.
Secondly, the term 'spectrum' is meaningful - we are all at different places in different aspects of our personality. Many of us show mild autistic traits without actually 'being autistic'.
Thirdly, use the diagnosis - if you get one - to be helpful. It can be used to unlock resources, to explain behaviour, and is helpful in understanding how best to approach learning and understanding.
It does not need to be a label that means the child, or adult, should always be taught or approached in a certain way or 'put in a box'. It is the person's own 'label' to help them and their family, and to be used as they wish.
Good luck to you all.