Using the dev version from today.
JabRef accept keywords in the keywords field without (or with space) separator.
By default biblatex use comma (,) as separator. Not using it causing some problems e.g. when you filter your bibliography by keywords.

Separators in biblatex can be modified. So I think it would not be the best solution for JabRef to check for comma only.
There should be a warning or something like that if you have multiple keywords but no comma between them.
If I understand you correctly you want to have a check that reports keywords={TestA TestB TestC} because there is no comma between the words, right? But what if the user decides to put spaces into their keywords like bibliographic manager, JabRef is the best (which would be two valid keywords)?
At least for keyword groups you can speficy your own keyword separator
You understood me correct and I understand you.
Currently I have no solution.
But keep in mind: A user of JabRef shouldn't know much about biblatex syntax rules. JabRef is a GUI. IMO it is the job of a GUI to keep the user away from "to complex things". You have to find a way to vizualize this different possible and valid ways to use keywords.
e.g.
You know how keywords/tags are displayed on StackExchange (e.g. StackOverflow?). Do it like this would be ergonomic, user friendly and easy to understand. This would protect the user for having problems.
I think this is a good idea. For crossref field we have such highlighting, so could be implemented similarly
I can second this, multiple websites provide multiple ways of keywords, unfortunately JabRef performs pretty poor in scraping them at the moment from these websites. Sometimes these are listed as keyword1, keyword2, keyword3, others as Key Word1; Key Word2; Key Word3 (please note the spaces) or Keyword1 [Tab] Keyword2 [Tab] Keyword3. It would be great if Jabref could automatically adjust the keywords into correct biblatex syntax based on these hidden/not so hidden characters.
I aggree with Sidlerchr and DOFfactory
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I think this is a good idea. For crossref field we have such highlighting, so could be implemented similarly