The next step on the stairway to the gold cap is the internet. Often times, WoW players forget that various websites can have good information. However, many of them are dangerous and just filled with lies or exaggerations. It is my advice that you stick to three main websites: Wowhead.com, Worldofwarcraft.com, and Mmo-champion.com. On wowhead.com, one can see average prices overall on every sellable item currently in the game. This is a nice start; however, most servers have economies that differ greatly. I played on one server where a +healing red gem went for 100g and another where the same gem went for 55g. Swings like this are just the beginning. Random drop epics, especially those BoEs from newer content sometimes differ by thousands of gold! (Consider this fact especially if you are planning on transferring, as this can be the greatest gold mine in WoW). Worldofwarcraft.com provides a little known list containing the most common bought items on the auction house across every server in an aggregate sense! This means that they average every server before listing the results. Unlike wowhead.com, this presents the reader with a much more accurate portrayal of what they may expect on their own given server. Results will be slightly different, but this tool should not be overestimated.
Finally, Mmo-champion.com provides the latest patch notes in the fastest and most reliable way possible. The information on this site is highly trusted and often sourced with Blizzard developers themselves. By knowing that an item such as infinite dust will drop 50% less, then you would know to stockpile it now while it's still abundant and then mass sell following the release of the patch. The greatest gold makers are the most informed readers! (I can not stress this point enough!).
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold. If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.