check CV before creating giveaway

That's the only official way to know about the reduced or zero CV status that I know of.

Steamgifts uses the US Steam store price (regular price, not on-sale price). So a $19.99 USD game will be worth 19.99 CV, unless it has been bundled at which point it will be worth 19.99 x 0.15 or if it has been given away free in large numbers at which point it will be worth nothing. But whether it has been considered bundled or free is only known by the asterisks.

sgtools.info has a tool to show the real value of already-created giveaways but again that does not help you to know the value ahead of time.

sgtools.info also has a "deals" page that lets you filter by SG bundled status, if you are looking to buy some games for full-CV giveaways. But I'd always be sure to double-check the price against the actual Steam Store because not all third-party key sellers use the same base prices.