As a jungler or as a player in general it'll become good to try and keep track of the enemy jungler. It's a lot to think about on top of your own thing, I know. But if you try to notice these things once in a while it'll become easier to learn to manage that one more thing.
It's not really any one person's "job" (You could say it's your junglers job, but I'd say it's more of a team effort) to keep track of the enemy jungler. But if you have any way of knowing their position then ping "careful"
You're mid and your opponent suddenly disappeared just as blue buff is spawning? Ping their blue.
If they gank top just double-click top lane on your mini-map and hold it for a moment to see how much health everyone has afterwards. When you let go of your mouse the map will snap back to you. Knowing the windows of when their jungler is either busy/away/low health as opposed to MIA for the last minute or so should be a big factor in your decision making in lane.
Mid laners. This is how you know which side to put your trinket on when you're out of wards.
Though tbh Skarner would be difficult to keep track of since he traverses his jungle so fast and ridiculously healthy right now.
Also, as stated before. Buying that extra ward when you back, and finding that moment to put it just a little bit deeper into the enemy jungle then your typical defensive ward makes such a huge difference.