If you're working with long boards, you'll probably want to make a set of winding sticks. But if you're working with shorter pieces, here is the method I use to remove twist and flatten a board.
Lay your board on a dead flat surface—I use my table saw. Touch one corner with your finger and then the opposing corner. Then do the same with the other two corners. If the board rocks with one set of corners, it's not flat. And depending on which corners are rocking, you can tell where the high spots are.
High corners will not rock, low corners will. Remove high spots with your handplane and keep checking the board on your flat surface. Sight along the bottom edge of the board to see where it is and isn't touching the flat surface. This will also reveal high and low spots. Continue to plane and check your board until it no longer rocks when you touch oppposing corners and when the bottom edges of your board lay seamlessly on the flat surface.
In addition, I use a backlit straight edge to check for flatness by laying it diagonally across the board in both directions, and along the width and length of the board, parallel to its edges.
These methods have always worked well for me and actually seem to work most effectively when I use them before cutting the joinery. Go figure.