If the block of chocolate you have is new, has shine and snap, and doesn't show "bloom", then the chocolate is tempered.
When I was doing chocolates, I invested in a tempering machine, so I've gotten lazy on hand tempering. But, the easiest method I found is to melt 2/3 the amount of chocolate to 115-120 degrees for dark chocolate. Add the remaining 1/3 (a large whole chunk makes it easier to fish out) and stir until the chocolate reaches 90 degrees, which is the working temp. Remove whats left of the seed chocolate. As long as you keep the chocolate in the range of 88-90, you can continue to use it. If it cools down, it can be reheated to that temp range, but if it goes higher, it will be out of temper. I used to hand temper 35 lbs of chocolate using this method, and it always worked very well. I've done small amounts in the microwave like this as well.
This method works if the chocolate you start with is new, tempered and has not been used before. If it has and is not in temper, then you need to melt to 115-120, add new chocolate to that, stir, cooling down to about 85, then reheating to 90.
Milk and white chocolate's working temp is 88, melting first to 110-115. Dark is 90. There can be a degree or two difference depending on chocolate brand, but as long as it's in the range you should be OK.
Hope that makes sense.