Personally, I would form such signs that I heard from dozens of interviews with gambling addicts:
the first sign is lying to relatives. A problem gambler lies constantly, he hides the smartphone on which he plays, does not give it to anyone. Also, when economic problems begin, the gambler begins to lie about all sorts of reasons for which he should be lent money.
Then health and sleep problems begin. Constantly checking the phone if it is betting to see the results of matches and how it is going in general. Stress and insomnia are the companions of any problem gambler.
I don't think I completely agree lying is a symptom of problem gambling or addiction to gambling/betting though. For example, I recall the first time I lost a significant amount of money (it was not much, but big enough for me to hurt) and I was asked why I was feeling down during those days after the loss, I did lie to my relatives and I did not tell them I lost money rolling dices on a crypto casino.
I have never considered myself to be addicted to gambling and yet I lied, why did I do it? perhaps because gambling is kind of a taboo topic here in my household and my family would have unnecessarily worry about me and my well-being.
One can lie when losing money, but it does not immediately means one is addicted to anything. However, an addicted person is more likely to say things which are not true in order to hide their habits from others who may judge and belittle them.
With the current state of the world and the morals of people changing to worse, lying is not even a symptom of anything, unfortunately.