Investigative questions

What basic investigative questions you would always have to satisfy in any civil investigation, regardless of the circumstances

Assuming by "Civil" you mean any issue not involving a crime:

That you have a positive ID on your client.

That your client will not use the information you provide for any illegal purpose.

That your client understands an investigator's job is a search for the truth and
any information that works against his/her case would also be provided honestly to a court hearing the issue.

That the client understands he/she doesn't get the investigative report until the bill is paid. :-)