To diagnose hyperglycemia of two weeks old the best investigation retrospectively is -

HbA1c can be used as a diagnostic test for diabetes providing that stringent quality assurance tests are in place and assays are standardised to criteria aligned to the international reference values, and there are no conditions present which preclude its accurate measurement. An HbA1c of 6.5% is recommended as the cut point for diagnosing diabetes. A value of less than 6.5% does not exclude diabetes diagnosed using glucose tests. HbA1c reflects average plasma glucose over the previous eight to 12 weeks . It can be performed at any time of the day and does not require any special preparation such as fasting. Ref - pubmed.com