[HttpPost]
public ActionResult MyAction(MyViewModel model, string password)

var user = await UserManager.FindByIdAsync(User.Identity.GetUserId());
var verifyPassword = UserManager.PasswordHasher.VerifyHashedPassword(user.PasswordHash, password);
if (verifyPassword == PasswordVerificationResult.Failed)
{
    ModelState.AddModelError("", "Password incorrect.");
    // If password is incorrect, ModelState will be invalid now
}

if (ModelState.IsValid)
{
    // save posted data
}