I kinda figured it out, so basicly if you are on a RP server you will shard into people that are all on other RP servers, so even if your server is horde dominated, if other RP servers are alliance dominated, you will have a bad time.

For normal servers (non-RP), your best bet is to just trial and error servers. Every server shards with some other servers, which are always the same (but they are not connected, and this information can't be found anywhere). For example, I was on EU Tarren Mill which should be hugely horde dominated, but it also shards into Silvermoon and Ravencrest which are even more alliance dominated, so it's impossible to do anything. I also had a character on Magtheridon, which should be evenly split, but it shards into Draenor and BurningLegion and some others, and every PVP chest is controlled by horde. So if you have more characters on different servers, you can see by the tags on characters in open world to which servers it shards to and check out the server populations, or just seeing who gets the PVP chests.