Need to confirm below
If we have ASA in multi context mode and it has only single interface say gi2 that is shared between 2 contexts and its subinterfaces have different
IP address then we can confirm that this is the outside interface of the ASA as a whole?
Well it depends.
I think by default the ASA is set to have same MAC address for each subinterface of a single physical interface.
To my understanding this is changed by setting the "mac-address auto" in the System Context configuration after which the ASA generates MAC addresses for the interfaces. The default setting is, as I said, "no mac-address auto".
Or you can even set the interfaces MAC address under the interface configuration mode.
Here is a link to a command reference which explains the command use
If the ASA only is connected through the physical interface Gi0/2 to the next device and its divided into subinterfaces then it would seem that one of those interfaces is used for outside connectivity.
Judging by what you say the ASA has a single Trunk link to some neighbor device and subinterfaces are used to handle "inside" , "dmz" , "outside" and all other kinds of interfaces for the actual Security Contexts.
Without seeing any configurations I dont know if there is anything else that can be said of the setup.