Dyfed-Powys Police are taking on proactive patrols of areas identified as possible sites for raves over the weekend as part of Operation Flamenco.
A statement said: "Spot the signs: unusual number of vehicles, especially camper vans, vans or trucks, seen locally; illegal trespassers may recce sites in advance of rave; people approaching landowners asking for land, in the guise of hiring it for acceptable activities.
"If you suspect anyone who approaches you for land might not be who they say they are, don't hesitate to contact police.
"Social networks make it easier for organisers to spread the word – rave attendance numbers can grow hugely quickly, and locations change quickly."
To contact police, call 101 or email firstname.lastname@example.org.
In an emergency call 999.