I would access the table varaible names and check if 'x' is a member or not.
T = readtable('patients.xls')
T = 100x10 table
LastName Gender Age Location Height Weight Smoker Systolic Diastolic SelfAssessedHealthStatus
____________ __________ ___ _____________________________ ______ ______ ______ ________ _________ ________________________
{'Smith' } {'Male' } 38 {'County General Hospital' } 71 176 true 124 93 {'Excellent'}
{'Johnson' } {'Male' } 43 {'VA Hospital' } 69 163 false 109 77 {'Fair' }
{'Williams'} {'Female'} 38 {'St. Mary's Medical Center'} 64 131 false 125 83 {'Good' }
{'Jones' } {'Female'} 40 {'VA Hospital' } 67 133 false 117 75 {'Fair' }
{'Brown' } {'Female'} 49 {'County General Hospital' } 64 119 false 122 80 {'Good' }
{'Davis' } {'Female'} 46 {'St. Mary's Medical Center'} 68 142 false 121 70 {'Good' }
{'Miller' } {'Female'} 33 {'VA Hospital' } 64 142 true 130 88 {'Good' }
{'Wilson' } {'Male' } 40 {'VA Hospital' } 68 180 false 115 82 {'Good' }
{'Moore' } {'Male' } 28 {'St. Mary's Medical Center'} 68 183 false 115 78 {'Excellent'}
{'Taylor' } {'Female'} 31 {'County General Hospital' } 66 132 false 118 86 {'Excellent'}
{'Anderson'} {'Female'} 45 {'County General Hospital' } 68 128 false 114 77 {'Excellent'}
{'Thomas' } {'Female'} 42 {'St. Mary's Medical Center'} 66 137 false 115 68 {'Poor' }
{'Jackson' } {'Male' } 25 {'VA Hospital' } 71 174 false 127 74 {'Poor' }
{'White' } {'Male' } 39 {'VA Hospital' } 72 202 true 130 95 {'Excellent'}
{'Harris' } {'Female'} 36 {'St. Mary's Medical Center'} 65 129 false 114 79 {'Good' }
{'Martin' } {'Male' } 48 {'VA Hospital' } 71 181 true 130 92 {'Good' }
varNm = T.Properties.VariableNames
varNm = 1x10 cell array
{'LastName'} {'Gender'} {'Age'} {'Location'} {'Height'} {'Weight'} {'Smoker'} {'Systolic'} {'Diastolic'} {'SelfAssessedHe...'}
any(ismember('X',varNm))
ans = logical
0