What are the signs indicating onset of wisdom because of meditation practice?

I think wisdom refers to insight into/ experience of

• suffering, it's causes and how to be free of it
• interdependence
• impermanence
• emptiness
• non-conceptuality
• no inherently existing separate self

and how to live a meaningful life according to it that benefits and liberates you and other beings.

Some features I experience(d) in meditation (shamatha awareness of awareness) and life that point into this direction (although it's hard to put into words and I might be wrong about thinking that it points towards wisdom, I'm not a professional):

• thoughts, emotions and so on just arise and pass in my mind, often I can just watch them without getting entangled with them, without seeing them as meaningful, permanent or „mine“ and without an immediate urge to act on them.
• pain (especially mental/emotional one) arises and passes, but it's not neccessary and inevitable to suffer from it. Awareness seems indestructible and there's always joy and equanimity that can be accessed in deep meditation.
• I'm not as attached to entities of the world anymore. I don't need them to find happiness or peace, since these are in awareness itself. Events in the world arise, pass and repeat themselves anyway, I'm kind of not that interested in them anymore (except if it causes suffering to others).
• 'Love' (or rather loving kindness) and happiness due to it come from inside me unconditionally, not from others being nice to me or attached to me.
• Happiness is happiness and suffering is suffering, no matter in which being they arise. I just don't want anyone to suffer, but to be happy instead, no matter their relationship to „me“.
• I don't really feel like having a real, permanent and separate self: Mental events rise and pass without being mine or controlled by „me“, I don't feel that much like having permanent character traits, I don't feel very connected to my past or possible future, „my“ body feels like an arbitrary vessel of conciousness, in deep meditation I feel like an abstract, spacious awareness (which is peaceful, vivid and benevolent). (although I have to admit that this feeling/attitude is also kind of strange and especially when talking about it, it feels/sounds a bit like a personality disorder)
• Often it's not neccessary and not helpful to put labels onto entities or beings, to put them into categories and judge them accordingly.

Becoming wise means having much understanding of essentials and thus exceptional foresight to avoid trouble.

One's behavior changes more or less gradually but changes are discernable immediately due to the kind of thoughts associated with knowledge.

For me, when I meditate regularly I'm not as easily hijacked by my emotions; I inhabit a calm place. While wisdom is a subjective term, operating from a calm center feels like a wise thing to do because I can observe what's happening around me without getting caught up in it.

You may find it helpful to refer to Visuddhiñana-katha/The Progress of Insight by Mahasi Sayadaw, which explains the signs that a meditator recevies from a Theravadin perspective. It's fairly comprehensive.

Personally, if I were to summarize it, you will reach a stage where you have a very deep understanding that nothing your mind craves in the outside world can ever be satisfactory because of annica/impermanence. This leads to a natural loss of desire for chasing things in the material world because they cannot bring lasting joy & a withdrawal of the senses inward. You'll also likely find yourself sticking to the precepts much more easily because you understand how breaking them creates the conditions for continued suffering. Like a child who sticks his hand in the fire for the first time and learns that fire burns; you won't want to engage in certain vices anymore. And then a bunch of other signs also, which is explained in far greater detail in the text I mentioned.

With wisdom there will be less suffering:

AN5.136:3.7: They’re wise. They have the wisdom of arising and passing away which is noble, penetrative, and leads to the complete ending of suffering.

Please note the "arising and passing" phrase. Suffering arises when there is clinging to what must pass. Although universal, suffering (and therefore wisdom) will arise individually according to the nature of that individual suffering.

For example, the urge to steal may arise, and one may mistakenly identify with that urge to ill effect. But that urge will also pass on its own. So it can be endured and relinquished. The wise avoid stealing. But the unwise do not.

In general, suffering is entangled with the limits of greed, hate and delusion. Right wisdom relinquishes those limits in a certain way:

SN35.72:5.1: “Good, mendicant! And regarding the eye, you will truly see clearly with right wisdom that: ‘This is not mine, I am not this, this is not my self.’

So wisdom grows in the individual according to how that individual relinquishes craving. Empty of craving, wisdom then becomes universal.

Traditionally it is described as "discernment". In simple words, you don't lump everything in black and white categories anymore, you see nuances. Your answers become much more "it depends" than before.

That's one sign. Another one is how you act. Previously, you acted out your frustration or disagreement as-is. Now you see: what's the point of getting angry if it won't improve things. You get more rational and less swayed by emotions, because you can see further down the road.

Another improvement is the expanded scope of your awareness. Previously you were aware of one topic you were thinking or talking about. Now you are effortlessly aware of a hundred other things and factors. You have a rich context and perspective on things. You effortlessly see "whereas", "therefore", and "on the other hand" of things.

The expanded scope of awareness allows you to no longer get stuck in a single interpretation. You don't get stuck in a negative picture of the world (or any other limited perspective). In Buddhist terms, you don't get stuck in a single world.