Flashbangs are a big help in a case like that. In a pinch spamming an HE grenade gives away someone's position if they take any damage from it. With any sort of grenade it's much easier to come out ahead.

With correct angles you both minimize the chance that he sees you first AND the angle between the two walls you're having to watch.

So in source using a wide screen resolution on a CRT (pretty common way to gain an advantage) The field of vision is about 110.

The idea in the example you mentioned is picking one side (In this case the left side which I'll explain) and sticking to it.
Moments before he may be able to see you the angle between the two walls is still probably under 85 degrees.

This idea comes up all the time in CS and it doesn't have to be a big difference between two potential spots you have to watch. Say you have a pistol and no nades and someone goes behind double stacked boxes for cover and could peak out on either side. You will die certainly if you aim in the middle at the box and hope to aim on him and shoot first. You are only slightly less likely to die if you oscillate back and forth between left and right since it's still 50% and theres a percent of the time when you aren't aiming at a possibility.

What is best to do is aim at the right side of the box head level. The reason is first you have to pick a side but second it is easier for right handed people to aim to the left than backhandedly to the right. Most people notice they can track much more smoothly right to left than left to right.

That maximizes your odds of winning that situation because at least if you guess wrong you stand the best chance to aim left and hit.

Back onto the angle problem you proposed I would watch the right side and stick to the left. but not become visible to someone on the left. If what I could see there was clear, I'd repeat it on the right side of the approach looking to the left corner. And since it's impossible to check everything but at least now I've seen enough I already know exactly where they have to be if they are on the left side. So then I would strafe out aimed there then rapidly as humanly possible make two semi sweeps of my pad (picking up the mouse twice to do so) to turn 180 probably while simultaneously strafing back to behind cover if I felt I was going to get shot. If he did fire I would then listen to the location and for any footsteps wait a short but random amount of time and strafe out aimed at the spot they came from pre-firing.

Lets say I didn't have 10-15 seconds to do all of that (bomb timer is 35 in competition) If I only had 12-20 seconds left on C4 I would pick a side and run towards bomb while moving up to the wall opposite the bomb site (to avoid exposing myself to someone around the next corner) while doing the swipes to check behind me just in case then continuing.

But angles in all situations are so key. If you approach the fork there from the middle of the street as you get closer and closer to peeking out the angle between what you have to check rapidly approaches 180 degrees AND you're not far enough from either wall (whatever side he is on) to minimize the sight advantage he will have over you.

The basic idea is by standing off from a wall/corner you need to go around the other person sees you for a lesser amount of time before you see them. If you plant your face directly into the wall, up to half your body can be exposed and be shot and you still cannot see the other person. now still looking directly at the wall. Move back from it 5,10,15 meters feet whatever is feasible you will start seeing the arm and then even more of the person who sees you.

Yes the game is still luck because you don't know (unless you cheat or heard him move previously) what side he is camping on. But with proper angles and approach and like these tactics you can maximize your odds. When you're playing on a CRT just that advantage there combined with optimal angles is enough to help you probably get the first shot even from a disadvantage like this at least against non-cheaters.

In most cases I would have a grenade and it wouldn't require such a meticulous approach and in public games this is totally overkill and not fun. But I don't really find source fun and haven't played it in about 1.5 years now.