Another hint: some expression of time or indication of a period in the past.
子供の頃
若い頃
小学生の頃
昔
結婚する前
去年
先月
Etc etc etc
To indicate that something was done frequently, add something like よく
独身の頃、友達と飲みに行くことがいつでも気軽にできた。
"When I was single, I used to be able to go drinking with my friends any time I felt like it"
園児の頃はとなりの子のうちによく遊びに行きました。
"When I was in kindergarten I used to go play with the kid next door a lot"
あの薬を飲み始めるまで痛みのあまりで夜寝られないことが多かった。
Until I started taking that medicine, I used to have many nights that I couldn't sleep because of the pain I had
Now, those could be phrased equally well without using "used to" specifically, but you see that the essence of "used to" is that it is a completed act or no longer operant condition, expressed with some term or phrase delineating the time or period of time.