Legal Question in Real Estate Law in California

First and foremost, it depends upon what you mean by "enforce." My answer assumes that you mean enforcement by legal process such as bringing a legal action against the individuals, and not creating a confrontation or other ruckus, which would be improper in any circumstance.

If you are the owner of the servient tenement (i.e., the easement is across your property), know the terms of the easement agreement, and are pretty sure the terms of the easement are being violated, sure. If you are just a member of the general public rather than an owner of the affected property, it's way less certain that you have a sufficient interest to warrant jumping in and attempting enforcement as a kind of "private attorney-general." Under the latter circumstances, I'd recommend consulting with an experienced attorney first, and have him/her check the recent case law.

Unless you have an interest as the property owner subject to the easement, I'd recommend leaving enforcement to the police or sheriff.