How do you know what you're doing is right?
Wow! That's a great question. Many long years of study and practice would be the short and not very helpful answer.
I'm working on exactly that with my students right now (class in Photoshop).
Step number one: Develop the language and ability to name what's wrong. You can't fix it if you don't know it's broke. Once you know it's broke you need to be able to explain specifically how it's broke. There's an implicit assumption here that you must embrace. Objective standards for broke and not broke exist and are applied by the professional community. To continue on this path assumes acceptance of those standards. Otherwise you can just say; looks great to me and that's all that matters and you're done.
Don't make this common mistake: If it's not broke......
Here's an example: This photo is a straight out of the camera JPEG. Can you put into words specifically what's wrong with it technically. That's step number one. Otherwise you're just blinding stabbing at it.
Step number two: Photography is a discipline mediated by high technology. That technology is increasingly complex and sophisticated. You have to master it in two ways. The most important way is to develop confidence in yourself so that you know you're right despite the technology. A Canon 5D mkII with an L series lens produced that JPEG above. That's over $3000.00 in hardware. The camera did a piss poor job and the photo sucks. I am fully confident that I am right.
Those first two steps don't come easy. It will take a lot of time and practice. It's really helpful early on to get support. I learned 40 years ago by latching on to a working professional and following him around. I gladly submitted to the humiliation of having him throw my photos into his waste basted one at a time after he told me what was wrong with them. I am forever grateful for his time.
Step number three is the other side of step two, mastering the technology. Help is available but it's especially difficult to get help in photography because there's so much bad help available -- even from sources that you wouldn't think to hold suspect -- like Adobe or Nikon or Canon, etc. The money in photography is in the consumer market and that consumer market skews the entire discipline. This problem surfaces for example in your camera that has an auto white balance setting. Auto white balance doesn't work (see photo above). It does work well enough for the consumer market and it's a required feature for that market. The camera manufacturers aren't going to admit that it doesn't work -- at least not publicly.
Adobe Photoshop has three one click options under the edit menu -- Auto Tone, Auto Contrast, and Auto Color. These options will never succeed in correcting a photo, but will frequently harm a photo severely. They're there because Adobe sells more copies of Photoshop to non-professionals. Adobe isn't going to warn you not to use them. If it looks too easy.....
So you need to do your homework and read and study and be suspicious of easy answers. There are folks here who can point you in right directions.
Step three B: Overread pointed you toward developing a good workflow. Really important. I'd flesh that out with this comment; The sequence in which you apply edits to a photo is critical. Don't try and color balance a photo that needs exposure correction. Don't sharpen a photo before you size it for output. You learn this from reading and asking for help and developing habitual practice.
Step three C: (Yeah, there's a lot to learn). Use the right tool for the job. For example if you have a photo that needs a Levels correction, and as a result appears too dark, you MUST NOT apply the Brightness/Contrast tool to the photo to lighten it. Applying the wrong tool can cause secondary complications. I see this all the time.
So full-circle then:
1. You have to be able to name the problem correctly: Photo needs a Levels correction.
2. You have to be confident that you're right.
3. You have to not get suckered into pushing the Auto Tone button.
4. You have to know what tool to use and how to use it.
Joe
P.S. First step then: What is technically wrong with the photo above?