In one single tree:
You want to fix two GTS Screws opposite each other in a tree trunk (e.g. Treehouse Set 2 Trees Dynamic). You can easily determine the same height with a tube level.
Find the common longitudinal axis with a compass (e.g. smartphone app): Transfer the compass needle (cardinal direction) from your already mounted GTS Screw to the drill, so that the alignment of your screw pair is almost perfect!
Mark the alignment of the screws (direction in which you have to drill) with a long ø 6 mm wood screw a few cm above the actual drill hole, this will then give you the direction later.
If you have several trees:
You can also work with a compass. Alternatively, stretch a string or rope between the trees to simulate the beam. You can then attach a right angle to this string to show the alignment of the GTS screws perpendicular to the beam.