How would I begin to analyse power in a spoken discourse text?

First of all you should look at the context to the given text which may give different speakers different types power, and keep this in mind throughout. For example, there is bound to be difference in the way a student speaks to a teacher, or a boss talks to their employee. Don't forget about your theorists here, such as Fairclough, Wareing, and the distinction between instrumental and influential language.

Then, whilst reading the text, watch out for imbalances in the discourse which can be shown through utterance length, interruptions and overlaps. Ask yourself, are the sentences active or passive? Where are the interrogatives and who is asking them? Who is stating the declaratives and imperatives? Are there any politeness features going on here? Remember that the powerful participant in the dialogue is the one who controls the discourse.