Which genre do you write?
Fantasy Writers dot org is a small site that has a forum and a story critiquing area that is only visible to logged-in members of the site. As the name suggests, it is limited to speculative fiction that falls under the fantasy umbrella, though any subgenre of fantasy or paranormal is allowed. Writers of all ability and experience are welcome.
The site is heavily moderated, and unhelpful, snide, or harassing critiques and forum posts are not allowed. There are some differences in the way the site is set up, and there are some rules (no more than one story or chapter submission per week, and the word count should be 7k or less) so there's a bit of a learning curve for using it to share stories, but its set up allows members to link chapters together so that a novel can easily be read and critiqued in order by other members. As with any other writing forum, the skill of the critiquers and usefulness of the comments is variable, but there are some good writers on the site. There are no restrictions about who can post stories for critique, but people who are active in the forums and as critiquers are most likely to attract readers for their work.
Asking if English is someone's first language can stop a pile-on if there are noticeable grammatical "tells" or if the poster uses phrasing or colloquialism that's considered harsh or offensive in English. It can also clue the poster in that they'Re being chastised for something other the quality of their work.
I'd agree with this, but it is a sensitive subject, and one where people can "assume" things based on the nature of someone's screen name. It can be perceived, at least, as a micro-aggression in some situations, even when asked with good intents.
The one time I asked someone this question while critiquing (because his sentence structures and grammatical errors resembled ones I've seen ESL students make with student papers, and others asked the same quesion), the person turned out to be vehemently and
proudly white. In fact, he was of a persuasion where being taken for anything but a white AMERICAN was seen as a grave insult. He was, in short, a militant racist (with a white nationalist facebook page to prove it). So I suppose the resulting anger and ragequit of the writing site (after a long racist flounce screed) was both amusing and welcome, but it goes to show that it's impossible to tell which languages someone speaks fluently based on their writing.