If I get bitten by a poisnous snake, how would i know that the venom got in my blood?
If you were unfortunate enough to be bitten by a poisonous snake there is no definitive way to know if you were envenomed. Tachycardia is a hallmark sign of envenomation but in this situation the person would be quite anxious and the heart rate would be elevated because of the anxiety.
The best thing to do is sit or lie down to decrease the heart rate (the faster the heart beats the quicker the venom will be dispersed if it is in the circulation) and of course activate the EMS system. When you arrive at the hospital emergency room they will probably go ahead and give you intravenous anti-venom because again, there is no sure fire way to know if the venom got into the general circulation.
If the snake bites you and draws blood, it is a safe bet that you have had exposure to the snake's saliva (and in the case of a venomous snake, its venom). Therefore, in the event of -ANY- snakebite, you should seek medical attention. Many venomous snakes look like non-venomous snakes. In fact, in the event of any animal bite, snake or otherwise, that breaks the surface of the skin, you should seek medical treatment as even bites from non-venomous animals carry significant risk of infection and exposure to disease. Rabies, for example, is only treatable if treatment is received shortly after you are exposed to the infection. Untreated, rabies is 100% lethal to humans.