How know if ModelState contains errors

Question

When a form is posted in my controller, I make the following check:

if(ModelState.IsValid)

If the model is not valid, errors are added to the <

Answer 1

In addition to Darins Answer:

In .cshtml:

@Html.Hidden("IsValid", Json.Encode(ViewData.ModelState.IsValid))

in JS

var isValid = $('#IsValid').val().toLowerCase() == "true";