Had a quick need for this today, so here is a one-liner.

PS C:\> Get-SPEnterpriseSearchQueryScope -SearchApplication fastquery |ForEach-Object {Write-Host $_.name, "-->", $_.filter}
People -->
All Sites -->
FileShares --> pedatasource:"File Shares"
SiteScope --> contentclass:or("STS_Web","STS_Site")
db --> meta.collection:db
sp --> meta.collection:sp

Of course, substitute "fastquery" with the actual name of your SSA.

For those query scopes that FAST cares about, you can't see the filter definition in UI.  So, this may be handy.

For those scopes not related to FAST (usually created in admin UI), there are no filters.

Yes, it is confusing.  And it will stay that way :)