I hope this is a question in good faith, as it definitely feels like it.
Generally, when looking for evidence someone is transgender, 3 key things are looked for. If gender dysphoria is 1. consistent, 2. insistent, and 3. persistent. That is generally what differentiates a cis person from a trans one. Sometimes gender dysphoria doesn't show up until late in life and sometimes it shows up the second they leave the womb.
When looking at younger children, there are signs such as calling themselves a boy when they are assigned female at birth, or picking to do more feminine/masculine things which are the opposite from their sex assigned at birth. The best way to explore this is to give them free reign in the clothes sections and toy shops to pick whatever they want.
However, it is up to the child mostly as they know what they are feeling best. If they feel like they are experiencing gender dysphoria (and are old enough to understand what it is), they probably are. If they have talked to you about it, then talk to the relative people (other family members, teachers, group leaders, etc.) and make sure they are comfortable in themselves.
In regards to the "woman on a trolley bus", does she need to know?