Well, for those of us who fine at English, we can tell. The hard part is for you to determine if you yourself are correct.

It's more useful to invite constructive feedback than to apologize preemptively; that's what I'd recommend people do. Add onto this that, culturally, Americans tend to appreciate such invitations more than they appreciate apologies. (I can't speak to other cultures; I'm about 99% certain there are some major world-spanning ones where this is not true.)

Don't.

I have noticed that people tend to jump on others language skills in an indirect attack on the person when its the idea they disagree with. For some it is easier to dismiss an idea they do not agree with by attacking the person than refuting the argument.

Working with many foreigners I have run into the apology style more than once, they have good standing and do not wish to be dismissed for merely bad language skills. I live here and my use of the language is atrocious.

I've noticed three classes of bad English. The apologetic folk, who will often try to explain things in multiple ways, or be extra-verbose, in an effort to get their point across clearly. Second, the unapologetically bad folk, who write a semi-incoherent sentence and respond poorly to requests for clarification (often simply repeating the same sentence). Third, the lazy native (or fluent) speaker. This is an entirely new class of errors, which is easy enough to read through (being native and lazy myself), but sends a strong signal that I shouldn't place too much importance on what I'm reading.