If I fit an SVM to imbalanced data (e.g. two classes and a sample where more than 90% belong to class 1 and only 10% to class 2) it may happen that the classifier simply classifies all cases as class 1. This problem can be handled by specifying costs and/or resampling, but I would like to know whether the classifier has such a simple form. Is there a way to check for this in a "simple" form (e.g. by looking at output objects in Python Scikit)?
I suppose the most straight-forward way of identifying such a problem is having a look at the actual predicitons of the model. Generating a confusion matrix of your models predictions is a standard procedure for this purpose and is implemented in scikit-learn.
Having a look at the example in the wiki article helps interpreting the confusion matrix, which tells us: it shows us that 5 instances of class 'Cat' have been correctly classified as such, while 3 have been wrongly classified as 'Dog'.
If your model is really struggling with the class imbalance in the data, then you will see most instances predicted as the majority class - many wrongly so.