Trevor's Script:
$siteUri = "[yourSharepointSiteHere]"
$proxy = New-WebServiceProxy -Uri "$siteUri/_vti_bin/ReportServer/ReportService2010.asmx" -UseDefaultCredential -Namespace "SSRS"
$proxy.ListSubscriptions($siteUri) | Export-CSV -NoType -Path c:\ReportsSubs_List.csv
Trevor's blog post to get more info:
http://sharepoint.nauplius.net/2013/12/quickly-identifying-reporting-services-subscriptions/