Background/Objectives: Polycystic ovary syndrome (PCOS) is a prevalent endocrine disorder affecting women of reproductive age. It is associated with glucose intolerance, insulin resistance and diabetes mellitus. The oral glucose tolerance test (OGTT) is commonly employed to detect glucose intolerance, but it can be inconvenient and time-consuming. We aimed to evaluate the precision of haemoglobin A1c (HbA1c) and other serum markers in predicting abnormal glucose tolerance (AGT). Methods: This diagnostic study involved 121 PCOS women who attended the Gynaecologic Endocrinology Unit at Siriraj Hospital. Patients underwent assessments for weight, height, waist circumference, modified Ferriman–Gallwey score and acanthosis nigricans. Blood samples were collected to measure fasting glucose and insulin levels after a 75-gram oral OGTT, fasting insulin level, HbA1c, lipid profile, androgen profile and complete blood count. Homeostasis model assessment of insulin resistance (HOMA-IR), homeostasis model assessment of beta-cell function (HOMA-β), quantitative insulin sensitivity check index (QUICKI) and fasting glucose-to-insulin ratio (G/I ratio) were calculated. The sensitivity, specificity and accuracy of these serum markers were compared. Results: The prevalence of AGT was 24.8%. The area under the receiver operating characteristic curve for HbA1c in detecting AGT was 0.656, while HOMA-IR, HOMA-β, QUICKI and the G/I ratio had values of 0.817, 0.737, 0.817 and 0.77, respectively. The G/I ratio cut-off point of 6% demonstrated a sensitivity of 73.3%, specificity of 74.7%, positive predictive value of 48.9%, negative predictive value of 89.5% and accuracy of 74.4%. Conclusions: The G/I ratio is the most accurate compared to other serum markers in detecting AGT among Thai women with PCOS.