Getting original time zone of DateTime value when parsing

I have a set of DateTime text representations in ISO 8601 format, some of them have time zone specified. By default, DateTime.Parse() adjusts them to the local time zone (or to UTC with a special option), but in both cases the original time zone is missing. However, I need to detect which of DateTime strings were specified with the time zone and get its value for further processing.

Any ideas on how to do that?

UPD sample inputs:

2015-06-26T22:57:09Z
2015-06-26T22:57:09
2015-06-26T22:57:09+01:00