Regarding　the　issue　of　network　covert　channels　hidden　in　overt　sources　with　fixed　probability　distribution,　two　detection　methods　are　proposed.　One　is　an　improved　attribute　shift　test　and　the　other　is　a　modified　Pearson　chi-square　test.　In　attribute　shift　test　we　use　k　warnings　instead　of　one　warning　to　indicate　detection.　In　chi-square　test　we　incorporate　adjacent　probability　values.　The　two　methods　improve　detection　accuracy　in　different　degrees.　Advantages　of　the　detection　methods　are　verified　by　experiments.