This is a huge area - it can perhaps even be argued that so very few texts are ‘really’ written or spoken that the majority of texts are blended in some way. That’s an extreme position, of course, but it does prove that mode is not straightforward and is worthy of discussion beyond just “x is in the written mode while y is in the spoken mode”.
Things to particularly consider about blended texts include:
The primary channel of reception and its effect (i.e. is it a visual or aural text)
How interactive the text is - is it monologic or dialogic
What is the relationship between writer/speaker and audience, and how does this manifest in the text
The degree of spontaniety or pre-planning and how that impacts the text
In electronic texts, does the lexis show features of ‘txt lang’ e.g. acronymisation etc
How formal or otherwise is the text in terms of lexis, syntax and structure?