In the chart above, the range is about twenty 15-minute bars long, so around 5 hours. A couple hours before a big news event, price will usually consolidate and trade in a relatively narrow range.
What will also help is to look at the previous session’s range. A lot of high-impact data releases are scheduled near the beginning of a session (e.g. Tokyo, London, U.S.). Since there is event risk ahead, traders in the session prior to the scheduled release will be hesitant to enter new positions. They will wait. The lack of trading activity is what causes price to trade in a range.
For example, let’s say NFP is coming out at 8/30am ET (the U.S. session). Take a look at the range of the London session. If you see a lot of candlesticks with small bodies and a bunch of dojis, and price hasn’t moved much, take a look at the high and low of the session, and you’ve got a range.
Plus, what if it is not even ranging? What if it is in a downtrend.
Trading a straddle is basically a breakout strategy. In order to trade breakouts, price needs to have been trading sideways and within in a tight range.
If price is trending, don’t try to straddle it. 
Be careful trading news though. The FX market is heavily manipulated and “traps” are common. For example, if the top and bottom of a range are obvious, the price will be pushed towards those levels simply to trigger entry orders, only for price to reverse and trigger your stop loss.
If you want to try it, use small position sizes like 25-60 basis points (0.25 - 0.50%) or lower.