An answer to a question on LinkedIn: Are other designers really the best critics of your work? The continuance of the discussion might be worth reading.
You ideally should have designer, art director, target audience and client feedback.
Client feedback is important because they hopefully understand their target audience and functional requirements better than the designer. Ultimately the client needs to be happy with signing the paycheck so their acceptance is important to the politics of the process. Though I keep reminding clients that they should try judge work from the perspective of their audience.
Target audience feedback is great when you get it. Clients are often open to some form of testing - and if you keep it lightweight it can be quite cheap. The thing is that target audience are often limited in the insights that they can give. They usually cannot see past an incremental improvement so take what they say with a grain of salt. Make proposals to users, don't ask them what they want because they can't tell you their latent needs. Cool design hits latent needs.
Art directors are great for judging the feel of a piece. Though in my small country often art director and designer are the same person.
Other designer feedback is critical. It's nearly impossible to critique your own work (though I'm actively researching techniques to make self-critique more effective). The arty side of designers might be fickle but the underlying design aesthetic is not as subjective as people think. There are underlying aesthetic principles that are quite universal and some that are culturally situated. Good designers can critique work because they are trained and have developed connoisseurship. They are not just somebody who thinks that just because "beauty is in the eye of the beholder" that somehow qualifies them as having a useful opinion just because they feel "creative". Also, the graphic design eye is very different to that of other types of visual creative.
Much of the formalist side of critique will encourage adherence to grids and greater coherency between things on the page in order to increase unity. If a piece is too unified/simplistic then the critique will recommend a "breaking of the rules" to create interest. That's the basis of most visual critiques (yes, I am researching this). E.g. On a website with many colours and type treaments, obvious critique is to choose a colour scheme and to systematise the type treatments because that increases unity. Counter e.g. On a website that is black and white, a critique might suggest to add a highlight colour in order to create interest.
To bring it back to the question: Designers are one of the important critiquers of your work but don't forget the others. I don't think there is currently a better way of measuring work where aesthetics is involved - though there are established tests for measuring function.
On a cautionary note: The whole "Design Science" movement tried and failed to objectify aesthetics in the 1960s. They thought that science could start to properly measure aesthetics - luckily we are over that though I do think that the post-modern reaction to design science swung the pundulum too far past humanism into outright mysticism where it concerns creativity. Contemporary techniques like fMRI are given interesting insight into how the brain interprets aesthetics - but fMRI is simply out of reach of most designers.