Erm, ePolicy orchestrator is not really the issue here, treat it as any installable program.

To Check the OS
Use @INWIN

To Detect if the agent exists, figure out what file the agent places on the machine, any file will do, as long as it is a file that would not be there if the agent was not installed. Then do an IF EXIST statement on that file

I cant help you with the log file, I dont use them often and have little experience.
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