how do I check which PI Data Archive I have?
Are you referring to the version? There's a few ways, the simplest is in Control Panel > Programs and Features.
Data Support Specialist
Mitsubishi Hitachi Power Systems Americas Inc.
Phone : 1 (407) 688-6840
EDIT: Looks like Sebastien beat me to it. Here are a couple other ways:
Using PI System Management Tools
PI SMT -> Operation -> PI Version
Using the pidiag command line tool
On the Data Archive machine, in the %piserver%\adm directory, execute pidiag -v
Both of these options will give you the version of PI Data Archive.
Retrieving data ...