Roll a second dice behind your screen
If you want the ranger to unknowingly roll with advantage then this is the simplest solution.
When the ranger makes his check, roll a second d20 behind your screen, then take the higher of the two rolls (his roll and your roll on his behalf) and give him information based on that, not necessarily the roll he rolled.
Your players will know something is going on when you roll a check but they won't know exactly what. If this is likely to be problematic at your table then roll a few d20's in advance of the session to use for this purpose and keep a note of the outcomes.
