Eventually, you'll want to get used to mentally tracking the position of the five enemies at any and every moment of the game. When someone disappears, you want to notice it, then ask yourself how long it would take them to gank you out of the shadows, assuming they started running full speed straight towards yourself. And then make sure you're near a tower before the hypothetical gank takes place.
In the mean time, just do a check of who's on the enemy team. If it's someone with a jump or stealth, try sticking a little closer to your teammates while roaming about.
And lastly, before going into any brush--and I do mean any brush at all--check your minimap, or press tab. If there's any opponents not visible on the map, assume they are all piled up in there, waiting specifically for you.