If you were contacted by a detective regarding a theft you are either under investigation, or simply a witness or potential witness that the police felt obligated to interview. There really is no way to find out if you are under investigation since the authorities are not obliged to let you know. The only way you could get a good idea if you are the focus of the investigation is if other friends, or co-workers have been interviewed by the police with much of the questioning centered on you.
The detective doesn't need to be honest with you regarding why he wants to talk to you or with any other information. Unless you were a witness to something which is likely being investigated by law enforcement, my assumption would be you are being investigated for something. Speaking to an attorney about your options is likely a good idea whether you are being investigated or not because statements to an officer (or anyone else) could be used against you later.