We have recently handled several cases for field service engineers. Many employers improperly classify this position as exempt. However, unless you are a licensed engineer working as such, you are likely entitled to overtime. Common misclassifications are employees who perform routine installations and maintenance of equipment in the field. These employees are entitled to overtime.
Another common issue in this type of work is the payment on a piece rate or flat daily rate. In either case, you are entitled to overtime for any time worked past 8 hours in a day or 40 hours in a week.
Travel time is also frequently an issue for this type of work. You are entitled to be paid for any travel time between job sites. In general, you are not entitled to travel time to your first job site or your last job site of the day, provided this can be considered part of your normal daily commute. If the travel time is for an emergency repair outside of normal hours, such as on the weekend, then you should be paid for that time as it is not part of any "normal" commute.