What are possible ways to unit test this method?

I am currently learning how to use the unittest module. I have a minesweeper like board as an object in the form of the class below:

class Grid:
    ''' class to represent grid sizes. '''
    def __init__(self, width: int, height: int, margin: int, rows: int, columns: int):
        ''' 
        width: int, in pixels
        height: int, in pixels
        margin: int, in pixels
        row: number of squares along y axis
        columns: number of square along x axis
        '''
        self.width = width
        self.height = height
        self.margin = margin
        self.rows = rows
        self.columns = columns
        self.grid = [[0 for _ in range(self.columns)] for _ in range(self.rows)]

    def gridDraw(self):
        '''draws the grid for the game board'''
        for row in range(self.rows):
            for column in range(self.columns):
                color = white.rgb()
                if self.grid[row][column] == 1:
                    color = green.rgb()
                pygame.draw.rect(screen,
                                color,
                                [(self.margin + self.width) * column + self.margin,
                                 (self.margin + self.height) * row + self.margin,
                                 self.width,
                                 self.height])

    def size(self):
        '''returns width, height, margin values '''
        return(self.width, self.height, self.margin, self.rows, self.columns)
    
    def gridVal(self):
        '''returns grid value'''
        return(self.grid)

My question is, how could I go about doing unit tests on this gridDraw method? It doesn't really fall under how I would normally test outputs with assertEqual() functions and the like. My test class is as follows thus far:

class GridTest(unit.TestCase):

    def test_gridDraw(self):

    def test_size(self):

    def test_gridVal(self):