The only definite sign of pregnancy is a positive result on an HPT. A missed period is a strong sign, but even that can have other causes.
Other symptoms are less reliable, because they can also be caused by normal hormone surges around the time you're due for your period. Some of the more common ones are fatigue, bloating and/or cramping, mood swings, sensitivity to smell, breast soreness, loss of stamina, and nausea/vomiting.
You might notice, though, that those are all very similar to the symptoms of PMS (and, if your'e pregnant, will come right around the same time). Hormones are hormones, after all. Pregnancy symptoms--particularly the fatigue--are likely to be stronger than your usual pre-menstrual symptoms, but not different enough to be sure until you've missed your period and/or popped positive on a test.
Also, you're unlikely to have ANY symptoms right around conception or in that two week period between conception and your missed period. Symptoms usually start around 4 weeks, though they can start as early as three. (So right when your period is due to make you feel that way anyway. Naturally.)