There are many ways to find the interesting points and sights of each destination. I personally always start with a good written guide, one of those without barely any pictures and tones of explanations. After the years, I have become to realize that I prefer the Lonely Planet. There are digital versions of these guides and it is always nice to have a backup on the kindle (specially because they weight nothing!). In any case, I rather buying them in paper for several reasons: in case I run out of battery, I don’t have to worry about not being able to access it, you don’t have to stress (a lot) over the effects of a sudden torrential rain may happen and… since after the trip I become quite attached to them, it is always a nice reminder of the trip and all we’ve been through together. In these guides, there’s usually information in detail about the weather conditions of the destination, basic social rules and warnings about usual scams. Besides that, there’s tones of information about sights and interesting points one must visit, the history behind them and the best way to reach them.
If you don’t like having books as much as I do, or you are just planning on going somewhere for a short weekend trip (even though libraries are very good options to get guides), the next best option is our beloved Google. During the last years they have implemented they search in a way that when we type “What to see in ____”, a lists with all the most important sights pop up on the top of the search. It doesn’t work for all destinations, but in general is a good idea.
In case that your destination is not on the system, another great option is TripAdvisor and then looking for “sights”. IMO, the best part of this website is that it can rank the places and sights according to the vote of thousands of people who have gone there. That same feature is not always the best: if a place is very touristic, many people will vote and we may find that its position in the ranking is way higher than it should be. But if you want a nice and general idea about places to visit and sights to discover, it’s not a bad idea.
To finish, another great option are forums and blogs. In this same website you can find a section with travel guides where I speak about both well-known places and hidden gems, from a different point a view. Also, a single google search will find you plenty of blog entries from people who’s travelled there and maybe, found something special. To finish, there’s some specialized forums (Lonely Planet Thorn Tree and TripAdvisor Forums), where it is possible to enjoy of the recommendations and comments of truly experts on the destination.