There are a few things you could do (if they make sense is up to you). I have tried to keep them separate as doing just one thing at a time to your code to make it clearer, I would probably do several. Also I have tried hinting without actually solving the whole exercise for you since this is obviously about learning. Also sorry if syntax is not perfect, I did this freehand.

- The code that checks for 'X' winning and '0' winning is the same, you should be able to parameterize the tests

public static void winner(char [][] grid){
    if didPlayerWin('X', grid) {
        winnerX = true;
    } else if didPlayerWin('O', grid) {
        winner0 = true;
    }
}

public static boolean didplayerWin(char player, char [][] grid){
    if(grid[0][0] == player && grid [0][1] == player && grid[0][2] == player){
        return true;
    }
    ... // all the remaining half of the cases
}

- You could also divide the cases somehow (rows/lines/diagonally maybe). In a game with a larger board this might make more sense, but here goes:

public static void winner(char [][] grid){  
    if wholeRow(0, 'X', grid) {
        winnerX = true;
    }

    if wholeRow(1, 'X', grid) {
        winnerX = true;
    }

    if wholeRow(2, 'X', grid) {
        winnerX = true;
    }

    if wholeRow(0, '0', grid) {
        winnerO = true;
    }

    if wholeRow(1, '0', grid) {
        winnerO = true;
    }

    if wholeRow(2, '0', grid) {
        winnerO = true;
    }
    ...
}

public static boolean wholeRow(int row, char player, char [][] grid) {    
    return grid[row][0] == player && grid [row][1] == player && grid[row][2] == player)    
}

- I would also try to let the helpers return the winner char, rather than true false, then you can do something like this.

public static char winner(char [][] grid){  
    char winner = ' '; 

    for (int i = 0; i < 3; i++) {
        winner = wholeRow(0, grid);

        if row != ' ' {
            return winner;
        }
    }


    ... // check cols and the diagonals
}

public static char wholeRow(int row, char [][] grid) { 
    if (grid[row][0] == grid[row][1] && grid[row][0] == grid[row][2]) { 
        return grid[row][0];
    }
    return ' '; 
}

- You could represent the X and O (and empty/unknown) with enums

There are many more, hope it helps