How do people determine the value of (pre-self-adhesive) sheets? Is there a catalog, website, oracle?
It depends on the sheet, but if they are essentially modern sheets, you take the CV for a single x the number of stamps on the sheet. If they are classic era (Pre 1940) then it's harder, but you can try to look at prior auctions to see what they are selling for.
If they are sheetlet (usually small sheets of 20 - 40), then Scott may have a CV for them. It also depends on what country the sheets are from.
You can check the ads in Linns for dealer pricing, but be aware that older US panes are flooded on the market, and prices are often a discount from face value as the panes are sold as discount postage. If you have a local dealer, check there as that dealer may be happy to sell you panes of stamps at or near face value.
edited June 28
The Brookman Catalogue and Official Blackbook Price Guide both quote prices for Mint Sheets but I've got to tell you that any seller on Hipstamp would be ABSOLUTELY DELIGHTED to sell to you at the prices shown therein. Brookman has been selling stamps for as long as I can remember, and I’ve been collecting for over 60 years. They have a webstore with an extensive inventory. I assume they must have some success selling at their listed prices.... on the other hand, I don't know how the editors of the Blackbook guide arrived at their “retail” values.
The recommendation made above to study the advertised Buy and Sell prices in Linns is a good one. Linns is a valuable resource for any number of reasons and worth the cost of a subscription. You will discover that some dealers are willing to pay a good premium over face to buy in quantity to support the needs of commemorative cover makers and other vendors. You may find it makes sense to sell part of your collection this way.
Of course, you should also look at the current offerings and sold results on eBay when setting your prices. You will find that sellers are often lucky to get $1.00 over face for some sheets. Many dealers open their sheet auctions at less than face value because they were able to buy at a step discount. As Michael says, the market for sheets has changed. People no longer collect full sheets like they used to