Is there a way to find out what decks contain what card?

Convert will convert the lowest quality card, if, say you have 3 non glossy versions of a card in your deck, but 1 prismatic and 1 glossy outside your deck, none of them will be converted.

To give more detail into batch conversion, there are two ways of doing it.

"Convert by Basic/Glossy/Prismatic," which will keep 3 copies (by default) of a standard card, another 3 glossy copies of the same card, and another 3 prismatic copies of the same, allowing you to keep up to 9 copies of the same card by default. Using this method, you will accidentally convert a prismatic card unless you already have more than 3 prismatic copies of a card.

"Convert by Card Name" will go straight down to the specified number, converting prismatic cards last. However, using this method, if you have 3 standard copies of a card in a deck, obtain a prismatic copy, and then convert, you will lose your prismatic copy.

Personally, I prefer "Convert by Basic/Glossy/Prismatic."

Batch convert (or even manual convert) can't convert a card that is part of any of your decks.
Also, in your case it won't convert any card. If you batch convert to 3 copies of each card, and you have 4, but 3 of them are in use in any of your decks and the 4th is not, but is of better quality (glossy or prismatic), none will be converted.