1 week before cycle. Most home pregnancy test can reliably detect pregnancy a minimum of 1 week prior to the date you expect your cycle to begin. Sometimes it is difficult to read the pregnancy tests with the lines, therefore, I recommend the digital pregnancy tests.
5 days before period. The earliest a pregnancy test can be positive is 5-6 days before when your period would normally start, or about 9 days after conception. Most of the over the counter pregnancy tests will be very reliable by the time your period would normally start. I don't know which brand name is the best, perhaps someone else can answer that question.