If I train a neural network on one dataset and finetune it on a new dataset, and the loss on the new dataset is low, how can I determine if the trained model is overfitting? Without a validation dataset during training, how do I assess whether the model is overtrained or not? I am not able to test on a validation dataset. Are there other ways to check if my model can generalize well?
You can't. That's the entire point of having a validation/test set.