There's no single, foolproof sign of pregnancy right at the start — each person's body reacts in its own way, and some of these signs also happen in other situations (PMS, stress, changes in routine). That's why it's worth knowing the whole picture, without treating any one of them on its own as proof.
The most common signs
A late period is usually the sign that leads most people to take a test. Breast tenderness or swelling, nausea (which can happen at any time of day, not just in the morning), unusual tiredness and needing to urinate more often are also frequent in the first weeks.
Changes in smell and taste, mild abdominal bloating and more unstable moods also show up in many accounts, but they vary a lot from person to person — some notice almost nothing until their period is late.
When to take the test
Pharmacy tests detect the hormone hCG in urine and usually work well from the first day of a missed period — testing earlier increases the chance of a false negative, even if you are pregnant. If in doubt, repeating the test a few days later or getting a more sensitive blood test (beta-hCG) helps confirm it.