Um. Half the fun is being on your guard when a person approaches when traveling.

As others have suggested; if you have a bow and arrows you can go in sneak mode and shoot an arrow a little to the side of them. If they turn red on the compass and start searching for you they are bad guys. This strategy gives you away somewhat, but if you've got a few perks in archery and high sneak it's relatively easy to take them out even if they are on to you.

Finding out who's hostile by firing arrows is not a good method ... some characters, such as some bandits and most giants, will not initiate combat unless you do something aggressive yourself, such as coming towards them with your weapon drawn. Unless you find that you are being regularly killed by hostile humans in the wild, it's best just to let the potential enemy see you and let them throw themselves on your blade where necessary.

That is kind of annoying, but I guess that really is how it would be, huh?

Just try an autosave.

You either wont be able too because an ENEMY is present or you'll have a save if you end up attacking someone you wish you hadnt.

Another option is to press the "wait" command and see whether the game allows you to wait or not. If you can't because there's an enemy present, you can always assume that it's either that dark, huge figure with the axe in front of you...or a mudcrab behind you that you haven't seen yet.

I like not knowing whether someone is friend or foe, adds a bit of tension to the experience.

If you have decent sneak and shoot an arrow on the other side of the enemy you won't be given away
not unless you are stupidly standing right out in the open 15 feet from them...in which case there is no hope for you go forward and let them kill you

I've lost a few companions this way. Poor Lydia died quite often by my fair blade.

I'm not too bad at judging people from a distance especially if they are in front of me but lydia would go AWOL for hours. I'd be out in the wild having some near death battles and just as i dispatch my last enemy i always have a quick spin round to see if anyone is still alive, there is Lydia sprinting towards me and my twitchy finger just gets the better of me. By the time I realise it's too late and I spend the next 5 minutes weighing up if I can be bothered to reload. I don't reload for my companions anymore luckily for him my character has a heart of stone and he can fully justify killing his friends if they get in the way.

I have this problem too. For instance, I wasted several invisibility potions sneaking past the people in the Ratway for the first time because I thought they were Thieves Guild members participating in an initiation hunt for the new guy. Didn't help that I could hear them say stuff like "Ha! Found you!" to skeevers. It was sometime later when I found out they were Thalmor goons...

I also almost attacked an imperial occupied fort ruin because I thought they were bandits from a distance.

That's rather clever. I never thought to do that. If they're enemies then it'll still alert them to your presence, but at least you'll be a safe distance away when they detect you.

I like the cut of this fella's jib.
Kill em all. Let Talos sort them out.

Yeah, not sure why red indicators only show up when detected, or your very close to them. Constant saving reloading to make sure I don't kill the wrong person, or to see if someone is an actual enemy or not.

I kill everyone who isn't immortal. Problem solved.