If there has been other traffic (which there will have been at this sort of site) then the easiest way to tell how soft the sand is to look at the previous tire tracks. If you can still make out the tread pattern of the tire, then the sand is firm (or at least was when they crossed). If you can see the tread pattern on the road, but in the sand in the tracks is largely flat on the bottom with no tread pattern, then it is getting soft - use your discretion. If the tracks in the sand look like V-shaped valleys, then stay out unless you really know what you are doing.
Can you give us an example of what you are looking at?
