I didn't quit experimenting, and this handler seems to work ...
on TrashIsEmpty(diskAlias)
quoted form of (POSIX path of ((diskAlias as text) & ".Trashes:"))
set sizeText to do shell script "ls -Rsk " & the result
set AppleScript's text item delimiters to {"total"}
set sizeList to (text items of sizeText)
set AppleScript's text item delimiters to {return}
repeat with i from 2 to (count items of sizeList)
try
(text item 1 of (item i of sizeList)) as integer
if the result > 0 then return false
end try
end repeat
return true
end TrashIsEmpty ------------------------------------------
This seems like something that Finder or System Events should be able to do. But, can they?