There is nobody to fall back on but yourself. You might be working with a programmers to execute your designs, but at the end of the day as a freelancer you are the sole person responsible for your own work. This means that if you give your client a less than desirable user experience they aren’t going to recommend you to anybody. Unfortunately they might even spread the word about their negative experience working with you. So take your time and make sure to go over all aspects of your project. Small-scale user testing throughout product design, or redesign, will help keep you on track and will allow an objective and fresh pairs of eyes to asses your design.