How do i know if I'm a good designer or my designs could be underestimated?
Design is naturally a highly subjective field so there may be no right answer to this question. However, a commentary on the technical aspects of your work might be more suitable for the format of this site. This will also be difficult from the one sample you've provided (although it looks like clean design work to me).
If I can take a different angle, I think what you might be looking for is confidence. This is something that can only be attained through a devotion to your craft, disciplined practice and industry experience (i.e; time). With patience you'll eventually wean out bad habits/practices and gain confidence that you're making the right decisions whether they're applied to the technical aspects of your designs or to the business side of dealing with clients. Keep in mind that being a good designer is also about knowing what the wrong answers are and how to avoid those pitfalls.
One final note: Doubt will always rear it's ugly head from time to time, but I always find the best way to deal with the "bully" is to confront it, defeat it, and put it in the proverbial rear view mirror. This too will serve to build confidence. Keep working hard and you'll have that designer swagger before you know it.
It's difficult to tell as you've only provided one example of your work. Looking at it, it's good. Better than a lot of crap out there. But it's not "wow". ("Wow" is not easy. It takes a lot of work.)
What separates good designers from bad ones is
Alignment and distribution
You seem to have to some level all of these in your card. I had a look at your portfolio on behance, your designs are good. There are few really good ones.
Reading your other comments, I can understand why your not being hired. Your designs have an ethnic feel to them. They also look a little similar, for example the colours are mostly reds pinks and oranges. You need to diversify a little more. People looking at your portfolio might feel your skill set is limited to this style.