Interviewers will usually make a deliberate effort to express a timeframe to hear back, and how to contact the company regarding follow up. However, if that isn’t mentioned, you should ask at the end of your conversation, when you have the chance to ask questions. Make sure to listen to the interviewer and abide to their expressed timeline and mechanism - if they suggest contact via email, don’t call - and listen to how they discuss timing. For example, if the interviewer says a decision will be made within a week, follow up in a week; otherwise ask if you can get back in touch at a certain later time frame to check in on progress, and keep good on your word and follow through.