I can't tell the difference now, but I used to could.

source: 
LAGS
Entry: 
647
Example: 
I can't tell the difference now, but I used to could.
Modal 1: 
used to
Modal 2: 
could
Gender: 
Female
Race/Ethnicity: 
White
Age: 
86
Level of Education: 
Some high school
Home Community, Town, or State: 
AR (Greenbrier)
Location Collected: 
AR (Greenbrier)
Year: 
1974
Relation to Addressee: 
subject to fieldworker
Medium: 
Spoken
Sentence Structure: 
1.1
Status: 
Naturalistic