ValidateModel(viewModel);

TryValidateModel(viewModel)

[HttpGet]
public ActionResult Index()
{
    var viewModel = new RandomVieModel();

    ValidateModel(viewModel); //For example

    return View(viewModel);
}

[HttpPost]
public ActionResult Index(RandomViewModel viewModel)
{
    if(!ModelState.IsValid)
    {
        return View(viewModel);
    }

    return RedirectToAction("SomethingNew);
}