Look at the imprint of the provider. Is the service provider located in UK or in a (European) foreign country? Even if the domain extension is .de, it does not necessarily mean that the provider is a Britan company. This doesn't have to mean anything bad, but it can make everything a little more complicated in the event of a dispute. While you're at the imprint: Why don't you also look to see if the service provider has any other businesses on the Internet. For example, someone who is both an editor and the head of a security company is either very good at both, or just mediocre at both.
Look at reviews. Decide for yourself if the reviews look natural or if they are bought.
Look at the a href="https://editius.com/resume-formatting-service/" rel="dofollow" class="rds_hl_outlinks"https://editius.com/resume-formatting-service//a if you use an intermediary agency. The mediators will send the texts to an external service provider - that's why it's so cheap.
For privacy-sensitive people and the legal departments of some companies, there could be a privacy risk here: You don't know who the actual processor is, what country they are in, and what happens to the data after processing.
Look for certificates or memberships in recognized associations and continuing education.
Pick up the phone and talk to each other. If you get along well with the provider, are satisfied with the way they work, and have a good feeling, that's already worth a lot. Book projects take a long time - so you need to get along well.
Compare quotes and check which services(like: a href="https://editius.com/paper-corrector/" rel="dofollow" class="rds_hl_outlinks"https://editius.com/paper-corrector//a) are included in the various quotes - and which are not. If the quote indicates that no sales tax will be charged, that's a sign that you're dealing with a small business owner. This makes small business attractive to individuals; businesses have no advantage (and in fact tend to have a disadvantage when dealing with large order volumes) when working with small business owners.
Proofreading at the touch of a button
The spell checkers of the individual word processors still have problems recognizing simple typos that result in correct words ("internet" and "boarding school", "roof" and "subject", "with" and "me", "online" and "inline", "coma" and "comma") as errors.
The algorithm also frequently has to pass when nouns and verbs are congruent. Duden Mentor does not detect an error in the sentence but a href="https://editius.com/resume-review/" rel="dofollow" class="rds_hl_outlinks"https://editius.com/resume-review//a does"Even with the congruence of noun and verb, the algorithm must fit frequently." The same is true for the current Duden Korrektor (as of April 2021 for both tests).
So with long texts, a human proofreader can definitely get to the result faster than the combination of, for example, the Duden corrector, which issues alarms or false alarms, and the author, who has to check these alarms.