How to find whether a dataset is blanced or imbalanced?
Typically, the representation of each class in a multi-classification problem should be equal. Say if there are 4 classes, then the ratio of count of samples in each class should ideally be n:n:n:n, most classification data sets do not have exactly same number of sample count in each class, which is fine and a lit bit of difference often does not matter. But if the difference is huge, say for example 100:5:9:13 then it matters and it is an imbalanced dataset.

coming to 400 GB of data to read - Depending on the type of your file, you can read it in chunks and then read and save the target variable( the one which has multi class labels) in another variable. 

You can visualize this variable (containing target) using a bar chart which will show you the count of variables for each class. Along with that you can also calculate the distribution of your classes to get better understanding of data.