[EnderMB]

You don't. In the grand scheme of things, it's probably a small minority of people that go public (i.e. publicly posting on the Internet) with their ideas for an app or business.

To answer your question in terms of code projects, I tend to turn to GitHub. More often than not, if I'm trying to build something and I've hit a problem a few selective searches through the code section will bring up something similar. It might be in a different language, but it's helpful nonetheless.

[yjm]

What I really don't want to do is lose a month implementing something and then realize that there was already something fairly popular out there that solved it.

I would rather not get too attached to my own ideas and tunnel vision implement them and ignore the reality that if there's already a good app for that I shouldn't bother.

[shivang]

You should have this in mind: Facebook was not the first social network, google was not the first search engine, amazon was not the first marketplace. So think of the market whether there is room for some more improvements and/or the market is huge enough to have more than one similar products. How can you solve the shortcomings of the similar product and be different.

[Mimu]

Following this point, I think that it probably exist (at least others people had the idea for sure, most probably didn't do anything with it, most likely for the same reason you don't), but if it's not big enough for you to know they exist, you're free to go imo.

Facebook was not the first social network, however if you want to make a facebook clone today you may aswell throw your money right away and save your time.

[EnderMB]

Echoing the other comments, think of it not as a missed opportunity, but as a challenge to build something better than what already exists. The odds are that anything you've worked on has (probably) already been done.