How can I know if newly released phone will be developer friendly? What are the facts to look for if someone want to buy a developer friendly phone (other than nexus phones) which means this phone is gonna have a lot of custom roms, mods and can stay up to date with the latest version of android for as long as possible with the help of custom roms.
A good starting point would be reading up on how cooperative the manufacturer of the SoC in the device has been with the (open source) Android community in the past. For example some GPU vendors have a bad reputation when it comes to providing information and the sources of the drivers for their products. If the developers/porters of ROMs have to go through hell to get stuff working, chances are it won't be very popular with the modding/custom ROM community, unless there are other very good reasons to persist (for example if the device is very high-profile, think Galaxy S or Nexus series).