How to know if your writing is any good?
The sad thing is — you can’t, at least not without help.
But readers do know what’s good and what’s not so good so to find out where your writing stands, you must find people to read what you write and tell you their honest reactions.
Of course, this only works if people read your writing who like to read the kind of thing that you have written. Otherwise, of course they won’t like it and they’ll probably be more than honest in telling you so.
But if you find the right people and they read what you’ve written and they like it, then you know it’s good.
If they don’t like it, probably they will have constructive criticism. If you listen to that criticism and make changes accordingly, then your changed writing will most likely be good.