//■ラジアンを返す

static float getRadian( Vector3 posbase, Vector3 pos1, Vector3 pos2 )
{
    Vector3 calA = pos1 - posbase;
    Vector3 calB = pos2 - posbase;

    float lba	= calA.Length();
    float lca	= calB.Length();

    float radian = (float)Math.Acos(Vector3.Dot(calA, calB)
                                    / (lba * lca));

    return radian;
}

Vector3 posbase = new Vector3(  0f , 10f , 0f);
Vector3 pos1    = new Vector3( 10f ,  0f , 0f);
Vector3 pos2    = new Vector3( 10f , 10f , 0f);
				
//■ラジアンを返す

float ans = getRadian(posbase , pos1 , pos2);

//■ラジアン→角度へ

ans *= 180f / 3.141593f ;

//■スプライトフォントで描画

spriteBatch.DrawString(font
                     , ans.ToString()
                     , new Vector2(20f, 20f)
                     , new Color(Color.White.ToVector4()));