The truth is that in most cases you, if you are not a native speaker of English, won’t be able to know with 100% certainty that the English is native sounding or not. That being said, take some time to reread your document with the eye of a reader (as compared to an eye of an author) and see how it sounds to you and then compare it to some sample articles from the journal you are planning to publish in.
I agree, this is a tricky one. Customers sometimes come back to us after submitting an academic paper edited by Uni-edit Editors to a journal, complaining that the reviewers recommended editing by an English native-speaker (i.e. that the paper’s English still sounded unnatural after our English editing). We take such claims seriously, and investigate each individually. Now, we have made mistakes before, but we often find that English flagged as “unnatural” is suitable, and that a reviewer was a non-native speaker themselves!
Even native speakers have bias about what kind of English is acceptable in academic English, and when they write “unnatural” or “non-native”, what they are actually perceiving is “this English is not written in the way I would have written it.” Some researchers, especially native speakers, simply lack the self-awareness and language knowledge to appreciate this difference (that does not make it acceptable behavior though).
Another common case is that a paper is rejected mostly for content-related reasons (e.g., insignificant results, unstable methodology), but a reviewer also comments on “language issues”. A good rule of thumb is, did the author comment on specific language issues, with lines and page numbers? If not, chances are high that the reviewer has a bias towards a certain kind of English and it is not the editing company’s fault. As a professional English Editor, I feel it is irresponsible and unkind to say there are language issues, without pointing out specific examples to the author.
If this happens to you on a paper that is Accepted with Revisions, here’s one solution: in your Response to Reviewers letter, state the paper was indeed edited by a native speaker of English and provide a certificate of English editing (provided by some companies). I’d say 4 out of 5 times, this satisfies the journal.
Another solution is if you have a colleague in your field who is a native speaker of English, you could get a second opinion: you could ask them to read the edited paper and let you know the quality of writing in general terms. If not, you have to judge the editing company based on past results. Did they edit a paper for me before that was accepted without language issues? Did they accept responsibility for mistakes they made in the past? This record of past performance should help you to predict the source of any problems. That being said, if you have too many problems, too frequently, you might consider changing which company you use.
I would recommend having a look at a section where the English is fairly standard, like the Methods section, and seeing whether the edited English looks like what you are used to reading in papers from good international journals. Secondly, if you have any native English-speaking colleagues, you could ask them to have a quick look at the finished product and see whether they feel it is well written. Finally, some journal editors may be willing to have a quick look at your manuscript before you submit it officially and to let you know whether the English seems appropriate for their journal or not.
I think if your paper is accepted by a journal, then it means the English editing quality is high enough.
I think quality is a subjective concept. In customer service, I have received a lot of feedback from authors. I got over 90% positive feedback. However, as an author, how do you know you received quality work? I think, first of all, verify all the edited texts are clear and matched to author’s intended meaning. Remember, beautiful, florid wording is not the journal’s concern, but rather a clear and meaningful expression.