GlobalFlag.m

GlobalFlag[name_String] := Module[{s},
  s = "Global`" <> name;
  If[NameQ[s],
    TrueQ[Symbol[s]]
  ,
    False
  ]
];

GlobalValue[name_String, default_:0] := Module[{s},
  s = "Global`" <> name;
  If[NameQ[s],
    Symbol[s]
  ,
    default;
  ]
];