There is really only one good way to assure that your minerals are evenly and well distributed in your mash...add them to your brewing water before adding the grains and mix them up. If you mix the typical brewing minerals in your water for a minute or so, they WILL fully dissolve. I've heard too many impatient brewers complain that they add minerals to their water and they just sit at the bottom of the container. Don't be foolish, while these minerals are fully soluble, it is NOT an instantaneous reaction. It does take a minute and it does help to mix the water during that minute.
The only mineral that doesn't dissolve very well is chalk and there is little that can be done to change that. For that reason, don't use chalk in brewing.