You can do a line trace.
What a (single line trace by objects in this case) is doing is simply trying to draw a line from a start point to an end point. You can get a random point around your character, add the highest your ground will ever be (for example 2k or something) to it, plug that into start and then subtract another 500-1k from it as end point.
You will receive a hit result with the location where the trace hit onto something. If you do it by objects you can define what should be hit (for example ignore trees / leaves while still coliding with other environment stuff.
I hope this helps.
Cheers