How to write correctly?
Where did you work before?
or
Where did you work in before?

Question

How to write correctly?
Where did you work before? 
or
Where did you work in before?

Answer

"Where did you work before?" is more correct.

Please also consider the options
- Where were you employed?
- Who did you work for?
- Who was your employer?
- Which firm did you work for?

Answer

'Where did you work before?' is better. The second one is incorrect.

Or even better would be 'Where were you working previously?'