Often your best bet is to land another interview, which gives you an excuse to call the original interviewer up and say "I'm interviewing for an opportunity at another company on [day] - Can you tell me whether you're still considering me as a candidate for [role] for which I interviewed on [date]?" If they say yes, you get the opportunity to ask follow up questions like "Great! Can you tell me when I can expect to hear from you?"
All that being said, I don't think you're likely to create problems for yourself if you simply call them and say "I neglected to ask you when I could expect to hear from you again regarding the results of my interview and next steps. Would you mind sharing that information, so that I don't accept another opportunity before you get a chance to respond?"
Note: If they say something like 'well, we're no longer considering you for the role', you have a huge opportunity to humbly seek some feedback for things you might do differently in future interviews. Rather than asking "what did I do wrong?" or "what didn't you like?", use your best I-really-just-want-to-learn voice and ask "Do you have any suggestions for things I might do differently to enhance my interviews at other employers?" You could get some very rare and valuable feedback that way.