Many higher end machines will indicate the degree of mineralisation on the meter. If you have a basic machine with no ground balance it won't be a secret as you will hear the effects and see the marked effect on depth when you dig the first target.
If you have a detector that doesn't provide ground information but does have a G.B. knob you can improve your knowledge of the conditions and the effect on I.D. and target depth by marking on the control box by the knob (or use a pre set arrow or a specific number) ie find an area thats easy to detect and where your getting really good depth. Mark the position on the control box or make a mental note that the knob is at say "6" on the control box.
Then next time you go to a sight and the GB setting has come about the 6 mark you will know that the ground is similar and you can expect similar depth. If you ground balance and the control is at 8 the ground is worse. This means less depth and on most machines the I.D. has already failed. If you have a meter, the needle, I.D. number or whatever will have dropped. This could mean that a fine gold ring that registers as say plus 4 on the meter on good ground would register as minus 1 or 2 so you would not dig it and miss a good find. If you have a depth reading on the meter you can apply the same thing. It varies from machine to machine but you can find out for every segment (up or down) you adjust from your marked point you subtract (bad ground) or add (good ground) a set measurement.
If on your machine one segment of increase on the G.B. above standard ground ie you have G.B.'d to "8" and each segment works out around half an inch inaccuracy then the depth reading is going to be one inch out.
Something to play with for those who like to experiment. You can do a similar thing with touch pad machines its just not as easy. Another thing to consider is that some machines like the Discovery Treasure Baron were designed were designed in a better way than more modern ones with targets in bad ground not dropping but rising. You still get an inaccuracy but the target I.D. remains in the plus side of the meter and so would be dug.