First I read through the whole text to get a general feel for the quality and style.
Then I compare the translation to the source text and look for omissions.
After these two initial tasks are done, I start going through the text sentence by sentence and follow these guidelines:
Are there any typos, misspellings, omissions or grammar errors?
Numbers and measurements – Are the numbers and measurements transferred appropriately for the target language? Are the conversions correct?
Consistency – Are the terms consistent throughout the text?
Fluency – Does the text flow in the target language?
Comprehension – How well did the translator comprehend the source? Does it show that the translator has expertise in the subject area?
Accuracy – How accurate is the translation of the terms and concepts?
Style – Is the style of the translation consistent with the style of the source? Has the translator used proper idiomatic language for the subject matter?
Audience – Is the translation appropriate for the intended audience?
Non-translatables – Are names and trademarks preserved correctly?
Diacritric marks – Has the translator used commas, exclamation points, semi colon etc. properly for the target language. The use often differs between the source and target language, at least for English and Swedish.
Based on these guidelines I can then make a general judgement of the translation quality and also give specific examples to the client, plus recommendations to the polish translator.