It is very important that your beer has reached Final Gravity before bulk priming. The residual sugars in your beer, if FG has not been reached, will produce high to dangerous carbonation levels in bottles.
The only sure way to know if final gravity has been reached is with hydrometer readings. Hydrometers measure the Specific Gravity of a solution. As the sugars are fermented out in the beer the SG will decrease. When the SG has not changed over a period of three days Final Gravity has been reached.
I usually take the first SG reading 10 days after the fermentation began. I take the next one three days later. If they are the same FG has been reached and I bottle when I get around to it. There is no harm in the beer sitting on the yeast cake a little longer.
Here is a link to using a hydrometer in Palmers old, "How To Brew" book. Also has a temperature correction chart to use if your SG sample is not the same temperature that your hydrometer is calibrated to. http://www.howtobrew.com/appendices/appendixA.html