When you are writing a dissertation and you know you have to get it right in the first attempt, you make all attempts to make sure your work is up to the mark and you have all the stuff in place before you make the final submission. However, when you are on it, it makes sense to ask a few people for opinions before taking the plunge. Here are some people you can go to in case you want a frank opinion.
Ask an elder: If you are doing a PhD and writing a dissertation, you are likely to have elderly relatives who have also treaded a similar path, before making your submission, you can have them check your work to give you informal inputs that can be achieved only with experience.
Other works: You can also refer to other similar works in your field and check old dissertations on the internet or in educational libraries. You may be able to lay your hands on masterpieces or who may be able to say, write one yourself!
Peer review: You can indulge in peer review services offered online, that help you get in touch with people with interests and educational backgrounds similar to yours and you may ask them to check your work so that they can point out mistakes or changes to better your work.
Divine intervention: When all else seems trivial, try taking a trip to the Himalayas, or simply meditate under a tree and ask for divine intervention, for all you know, the God almighty may descend to answer your prayers and grant you a successful dissertation in the process.
All in all, it is important to know that a dissertation is truly complete only when it is double checked and an opinion about it is taken from multiple numbers of experts from the field.