Use ticker symbol search for token pair (ie BTCUSD), find exchange that corresponds to market question. Navigate to Full-featured daily chart, Opening price is determined on the date in the market question on tradingview.com.
Opening price can also be found on tradingview using the hourly chart for the date in the market question at UTC (0) 00:00
If the trading pair market isn't available on tradingview.com, refer to the actual exchange. For example, if Coinbase's tradingview.com data feed is unavailable, find the opening price on Coinbase's exchange by using the hourly candlestick chart adjusting for local timezone offset. In order to find equivalent 00:00 UTC-0 hourly candlestick for December 16th, go to hourly candelstick for 00:00 December 16th, then count backwards or forwards the number of candlesticks depending on local time zone offset. If local timezone offset is UTC -5 move back 5 candlesticks to find the Open Price for 19:00 December 15th hourly candlestick.