The best time to check would be the first missed period. If negative, a delay or difference of 1 week is acceptable for periods to occur. Repeat the test after a week and start taking folic acid supplement tablets if trying for a pregnancy. Most young couples 85 percent conceive spontaneously after a year and 95 percent after two years.
I also noticed that you have type 2 diabetes on insulin and metformin. It would be wise to attend a preconceptional Gynaecology clinic to make sure your HbA1c is below 6, medications is okay and blood sugars within normal range prior to planning pregnancy. Although recent data suggest metformin to be safe in early pregnancy it would be wise to adjust dosage of insulin so that multiple drug therapy would be avoided. Insulin is very safe in pregnancy, but requirements would fluctuate as pregnancy progresses.
Do let me know if there are any further queries.