Free counsel is worth little. Unasked advice is worse. Folks even consider that you have a need to talk and they do you favour* with their time to listen.
To intervene spontaneously is kind but risky intruding, seldom appreciated. Counsel is critique, those who need it most like it the least. To face critique requires courage . Weak people, and most people are weak, misunderstand the generosity of spontaneous counsel; the one taking risk to intervene is a fool for them and a fool – even a wise one - is not credible. The weight of advice comes from authority, wealth, power, celebrity or success more than from insight.
If you want everyone to respect your advice make them pay for it with some sacrifice or why not, with money and formal respect. Let them make an effort to obtain it. To have your judgment followed, cause people to ask for it as they ask for salt; postpone, wait for the felt need.
Make your competence visible but do not push it. Great things are visible from far, like mountains, but they do not move. Let people come to them.
As there is a time for everything, there is one occasion for giving advice and many times for keeping silent. The right time to speak is when it hurts, and the person suffers, lost or sunk, thrown in the pit, with little way out. It may be necessary to wait cruelly and watch one going down until they are ripe to follow counsel. Earlier warning could have avoided the pain.
The signal that people are open to advice is simple: they ask for it or show despair.
All this cheap wisdom is the human comedy of advice-giving. I know its truth but alas, I do not have the heart to be wise in this way. Giving counsel to other people was one of my strengths , because I was bold enough to tell the truth to harsh individuals everybody feared and lied to and I allowed myself to intrude, for compassion and sense of justice, when weak people were in need of someone to tell them what to do. I was paid for this, with respect by the tough and gratitude by the needy. Occasionally, the stupid misunderstood and the mean took advantage of me.
The best I can do is to keep in balance being generous with being professional.
* As one who earned his bread, quite comfortably, as a consultant well paid and occasionally respected for advice in difficult situations, I keep being surprised when I encounter this attitude; I believe I am generous, while the listener believes that I am boring.
 I must also confess that like Alice in Wonderland I often gave very good guidance to myself though seldom followed it. That helped me evaluate how helpful it could have been: “She generally gave herself very good advice, (though she very seldom followed it)” Lewis Carroll , Alice in Wonderland, Chapter I.
 This is how I learned to look into one’s eyes and say with a casual air “This reminds me of a story...” Stories are examples that carry advice, as if for somebody else.