All very good advise.  I dug a a construction site in Lincoln Co. from July through Christmas. It was never posted and I only went on Sundays while no one was working. A couple times some workers showed up while we were there. I showed them what we were finding (quartz crystals and rutile crystals) and gave them a few specimens. The next time they came while we were working on a dry dusty day they brought the tanker truck over they were watering roads with to keep dust down and soaked the area we were digging so that we could easily spot smaller stuff we had missed in the tailings. They even started to keep an eye out and tell us when they saw rock that looked similar to what we were finding quartz veins in. It pays to use common sense and respect. We got to dig a long time and found some nice specimens with no worries.