Being able to pick up new skills is an important talent for a developer. It's unlikely that you'll be using exactly the same technology if you stay in a job more than a year or two. If you have become proficient in some technologies, then you should be able to learn others and apply the skills and methodologies you've learnt to new languages or frameworks.
Things to ask at interviews would include not only about the specific technology you're using now, but also about the company's policy towards training and development. Lots of companies might be more committed when interviewing than when you come to request time to do a course/conference/training, but if they're not willing to talk about providing training in an interview that's a red flag.
In the workplace, you need to learn how to discuss your skills with management and team leaders, to admit when something is outside your expertise, and to request help from colleagues. A good manager will have some idea of your skills but may assign you tasks outside your comfort zone to encourage you to learn and develop. You also need to know how to manage your career development and training needs as you progress: both when you start and later in annual appraisals or as needed.
Finally, it seems you may have confidence issues. Remember, if you're honest at the interview, then you will be hired because the company believes you can do the job. You should remember that. If you have good, supportive managers that have faith in you, then that makes it a lot easier, and that's another thing you should pay attention to at the interview.
