If I'm not given any specific feedback(And especially if I think the feedback given—when present—is stupid), my usual response is...
Sounds to me like you're not a good reader.
And yes, I have said something to this effect to a review swapper whose work I dropped partway through under penalty of being boring(to be clear, I dropped my review first and told him to not bother reviewing my work. He persisted, going so far as to read far more of my work than I did his.). He apparently talked about it in passing in the forums after, even though he admitted English isn't even his first language. I'm not naming names; it didn't even happen on Tapas(and thank god for that!).
I'll touch on a different track than most of the people here(actually, two!): How does one decide they are good or not if they can't trust review swappers to give them an honest assessment? Multiple-choice, coming up!
Post it on an online website, get review swaps with literally anyone who'll listen, and hope you don't end up saddled with a bad reviewer who's going to sandbag your ratings unfairly.
Recruit friends you trust as beta readers to have a look for you, just read through and tell you what they think and what they think could be done better
Go read a few works from published authors(Preferably physically published books), and compare your work against theirs(and maybe take notes of cool prose tricks you'd like to try and emulate).
Option one is for the naive and the incredibly hopeful. Option 2 is excellent, if you have the right friends on board (but it can be really, really slow). Since I'm something of a foolish bastard, though, I've recently taken to door number 3.
For those who took door number 3, please stop thinking that comparing yourself to Rowling or King is even remotely a fair right. Published authors have had probably at least half a dozen different revisions on their work, AND had professional editors buckle their work's grammatical shoes for them, AND had the same editing house people telling them NOOOO on certain things that they didn't catch on to as being foolish.
That being said, there is a pretty good benchmark out there in the far reaches of door number 3: compare yourself to the usual gang of clowns on authortube that actually published, and figure out if you think you're confident that you can outdo some of them, because if you are, you're actually probably fine. As for your first opponent to go up against, I recommend looking at this review of a book written by Jenna Moreci:
Keep in mind, Jenna's been successful despite writing pretty subpar work (mostly due to her marketing more than anything else) and despite fancying herself as skilled enough at the trade that she routinely gives writing advice in her own Youtube videos(many pieces of which, hilariously, she failed to follow in her own works). If you can tell you're not as much of a bumbling fool as some of the clowns on Authortube(Not just talking about Jenna here), you're probably a pretty decent writer like me.
If you're still not sure, though, keep in mind that there's downright hot garbage on Wattpad that somehow manages to get famous there(as well as some pretty subpar stories in their premium selection). If you want to make it as an author, you can probably get away with not being so hot at writing if your marketing's strong enough(or, alternatively, if your editor is a literal god)
Short of it is, maybe you are, maybe you aren't. Don't worry about it too much.