Detecting 16,32,64 bit modes

In a windows related enviroment I am trying to detect if a code section
I am attached to for debugging(dissembling) is running in 16,32 or
64bit mode.
(I am trying to avoid API calls and such...but if it is unavoidable...)

At first I thought it was easy just reading prefix 66 codes etc.
However, as I'm sure you are aware these prefixes are just toggles for
whatever mode the segment tables say it should be running in.

This is where I fall on my face.

How can I safely detect what mode another process/thread/etc is running
in when I am debugging(dissembling) it.

***
I am writing a single-step debugger to bring myself up to speed with
x86 programming.
***

Many Thanks,
Moldy

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