----------------------------
-- returning a string
----------------------------
set mounted to do shell script "test -d /Volumes/username && echo yes || echo no"
if mounted = "yes" then
	display dialog "Is mounted." buttons {"Ok"} default button 1
else
	display dialog "Not mounted." buttons {"Ok"} default button 1
end if

----------------------------
-- capturing the exit status
----------------------------
set not_mounted to 0
try
	do shell script "test -d /Volumes/username"
	
on error number errorNumber
	set not_mounted to errorNumber
end try

if not_mounted > 0 then
	display dialog "Not mounted." buttons {"Ok"} default button 1
else
	display dialog "Is mounted." buttons {"Ok"} default button 1
end if

----------------------------
-- using a boolean
----------------------------
set mounted to true
try
	do shell script "test -d /Volumes/username"
	
on error
	set mounted to false
end try

if mounted then
	display dialog "Is mounted." buttons {"Ok"} default button 1
else
	display dialog "Not mounted." buttons {"Ok"} default button 1
end if

open check-volume-mounted in the applescript editor