Usually, the company will put in their job description if travel is required as part of the job. If they did advertise travel is needed, you should realize that going into the interview. If they did not, you will have to decide if travel conflicts with any outside obligations you have, such as childcare or higher education. Be honest with the interviewer, and explain that you are available to travel if hired, but if you have to make arrangements for something, let them know you would need a bit of notice. The more flexible and agreeable you are, the better it will look to the interviewer.