Draw 2 lines; one at the lows and one at the highs of the 1st 15 - 20 min. If the market can stay above the 15 min high candle bar, the it is considered positive and the gap could possibly hold up for the day , however if the market falls below the 15 min low, then the likelihood of the gap fading increases greatly.  Realize that this is just a technique only and doesn't work all the time!

here's an example, the 7/21 EMA crossover is not part the gap rule, they were simply on the chart

gap example.png

But again, also gauge how overbought or oversold the market is per my comments above in the sticky and are you gapping up over a trendline or to resistance etc