1: Research, to find a cache you have to go where there was someone in the past that had enough to hide(wealth), location, analyze the terrain, think where YOU would hide something, check old records regarding the area of interest ...list goes on
2: You mentioned a DFX, well you need to practice a whole lot with buried items at different depths and settings on your machine. You are learning a new language thus learning to understand what your detector is trying to tell you.
I have a program with slow sweep speed, pre amp of 4, low discriminator settings. I run the DFX on MIXED mode that way I dont miss deeper targets. The MOST important way to tell if it may be a cache is by sizing the object. This is performed by measuring the signal intensity with the trigger pulled in (all metal or pinpoint). You approach the target and mark where you get the best reading ( smaller number or less depth) then mark where the signal starts dropping. Repeat same procedure in another direction and you have a pretty good estimate of the size and shape.
Remember that sound lenght does NOT mean anything in relation to size of object.
The discriminator only helps if the object is shallow and even then may be inaccurate as the container holding the cache may be of any material.
Almost forgot, a bigger loop will help substantially...I am using an 18" DD coil that makes a huge difference for larger than coin targets.
If you are not used to the machine and programs, then Relic mode will help you for now.
Gilbert