Keeping test data private is good practice actually. It keeps from hyperparameter optimization on the test set. Yes theoretically the dataset authors might stil

Keeping test data private is good practice actually. It keeps from hyperparameter optimization on the test set. Yes theoretically the dataset authors might still do it, but it's still better for the field as a whole to keep some test data secret and only use it for evaluation. Registering before getting data is common, so they can make you agree to some terms. It's not suspicious in itself. There are no hard and fast rules of thumb. You can look at the group, the people involved, the country, the writing style, the quality and thoroughness of ablation, is there code available etc. But nothing is 100%.