The problem in identifying addition prone candidates is a problem of ethics. The primary behavior that defines "the gambler state", and many other forms of addiction, is a neurological disorder related to responses of dopamine flooding. This is a physiological condition in the brain that can be observed, validated, and analyzed against trends.
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The addiction
In the most simple of terms dopamine is a brain hormone related to pleasure responses. If you eat chocolate and it tastes great your brain might get a small reward of dopamine. This is normal behavior and it is intentional so that the brain learns that pleasurable responses follow good decisions that result in rewards.
The part of this behavior that triggers addiction is that the dopamine response is not consistent. Planned and known wins result in small rewards. A surprise win results an observably larger reward. Again, this is proper human behavior intended to influence learning and decisions in the brain.
Addiction sets in when there is a disconnect between the dopamine cycle (the pleasurable stimulus from a win) and the resultant learning. A healthy person learns that the surprise win results in greater pleasure and modifies their behavior to find or achieve other surprises, because the current win is no longer a surprise and will no longer trigger the big dopamine rush. An person suffering from addiction, however, repeats the same behaviors in order to reproduce the same surprise wins because their brains haven't learned from the surprise. In their case their behavior is like-wise modified as influenced by the dopamine response, but instead of influenced in a direction for making future decisions they are stuck in a state of instant replay.
While the learning dysfunction is bad it isn't the primary problem of addiction. When the cognitive decision making is informed by a short-circuited response to a non-cognitive physiological condition of the brain the person won't know there is anything wrong or impaired. After all, either way there is a dopamine cycle and modified behavior. It is impossible to determine the impairment yourself because that very level of awareness and reasoning are influenced by this dopamine-learning cycle that is impaired in the first place.
That impossibility is the problem of addiction.
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The ethics
People with the dysfunctional behavior are identifiable. The problem is what do you do with them. You already know they are subject to a hopeless cycle of instant-replay and can use that knowledge to bleed them dry. Their decision making on a very foundational and emotional level is hopelessly damaged.
On one hand you can ask them to give you everything they own, and they will if you ask them correctly. This isn't just gambling or addiction, but qualifies a whole host of behaviors associated with marketing generally and isn't necessarily wrong... immediately.
Ethically speaking there is nothing wrong with persuading or manipulating people. People encounter such choices numerous times everyday. The ethical part creeps on two fronts:
1. If you know somebody's basic decision making capability is damaged and you intentionally abuse that damage to your self satisfaction
2. If you know bombarding a person, whether or not in a state of reasonable deciding capacity, with messaging to influence that person contrary to their objectives or in excess of a reasonable period of messaging
In many cases things that are commonly referred to as vises intentionally violate those two identified points of ethics. The legal available of a service or product does not qualify the ethics around such.