if($QuantityToAdd > $ProductStock){
   // throw error
}
else{
   // add to cart
}

if(($QuantityToAdd + $QuantityInCart) > $ProductStock){
   // throw error
}
else{
   // add to cart
}

if ($this->checkoutSession->create()->getQuote()->hasProductId($productId)) {
    //product is available in the cart
}

$this->cart->addProduct($productToadd, $params);