How to come to know that you have done some thing non-trivial in the research?

In a recent project I felt that I had somewhat trivial results, and that I needed to add more to it before writing the paper. The new steps turned out to be so non-trivial, and take so long time, that I decided to write up what I had in the meantime. By putting the work so far in the form of a proper paper - you know, with an introduction that attempts to convince others that the work is interesting and worthwhile - I ended up convincing myself that the results were non-trivial and made for a good paper already.

In hindsight I had worked on related stuff long enough that my results seemed less novel to me than they would to another person. So, my advice is to write it up, and trust your supervisor.

It's not surprising that the work of a Ph.D. student "seems less significant" than that of "top researchers". That seems an unreasonable comparison. The reason those guys are top researchers is exactly that their work is (usually) better than what the rest of us produce. That's not a reason for all of us non-top researchers to stop publishing. If your adviser says you've got something worth publishing, then write it up and publish it.

Two points worth making:

Firstly, that there is a huge range of published papers out there. Some are ground-breaking, while most are incremental. The incremental are still worthwhile.

Secondly, that when you have been working on a problem for months or years, it's easy to lose perspective. Because you've been absorbed in it, it starts to seem trivial or obvious, but to people who aren't intimately familiar it may still be new and interesting. The best way to regain this perspective is probably to talk to your colleagues, but you could also consider presenting at a conference (being careful not to prejudice future journal publication).