Reasonably high revenues in proportion to amounts you are playing for; reasonable withdrawal limits
If you happen to win a huge amount of money in a small casino, it will cause quite a disruption to its cash-flow. The casino's motivation to act dishonestly will be very strong in such a case. In the worst case scenario, the casino will face insolvency. Then it is not about honesty or dishonesty, but rather about them simply not having the money.
Some slot machines have such a high variance, that you can win up to 5000-times your bet in just one spin. In such a case, by betting €50 you would win €250.000. Casinos do not like setting the betting limits on their games, so that they don't discourage the most affluent players. More transparent casinos bypass this by setting a maximum limit for withdrawals. In such cases, you at least now see that a casino doesn't have high revenues and a truly high win would take you years to withdraw. The least responsible casinos do not address this, and only hope that no one wins such a large amount.
In any case, before you start playing, estimate the financial strength of the casino. Always count with the best - with slot machines be sure that the casino can pay out even a win of 5000-times your maximum bet. If you by chance win a large amount and the casino does not pay you off, it will most likely impact your mental health.
Definitely also check the withdrawal limits. It would take you nearly 7 years to withdraw €250.000 by receiving €3.000 monthly. That is enough time for the casino to go bankrupt, and it would not pay you the rest of your money.