You could use the streamer plugin to detect if a user is in a certain area.
pawn Code:
native CreateDynamicCircle(Float:x, Float:y, Float:size, worldid = -1, interiorid = -1, playerid = -1);
native CreateDynamicCylinder(Float:x, Float:y, Float:minz, Float:maxz, Float:size, worldid = -1, interiorid = -1, playerid = -1);
native CreateDynamicSphere(Float:x, Float:y, Float:z, Float:size, worldid = -1, interiorid = -1, playerid = -1);
native CreateDynamicRectangle(Float:minx, Float:miny, Float:maxx, Float:maxy, worldid = -1, interiorid = -1, playerid = -1);
native CreateDynamicCuboid(Float:minx, Float:miny, Float:minz, Float:maxx, Float:maxy, Float:maxz, worldid = -1, interiorid = -1, playerid = -1);
native CreateDynamicPolygon(Float:points[], Float:minz = -FLOAT_INFINITY, Float:maxz = FLOAT_INFINITY, maxpoints = sizeof points, worldid = -1, interiorid = -1, playerid = -1);
native DestroyDynamicArea(areaid);
native IsValidDynamicArea(areaid);
native GetDynamicPolygonPoints(areaid, Float:points[], maxpoints = sizeof points);
native GetDynamicPolygonNumberPoints(areaid);
native TogglePlayerDynamicArea(playerid, areaid, toggle);
native TogglePlayerAllDynamicAreas(playerid, toggle);
native IsPlayerInDynamicArea(playerid, areaid, recheck = 0);
native IsPlayerInAnyDynamicArea(playerid, recheck = 0);
native IsAnyPlayerInDynamicArea(areaid, recheck = 0);
native IsAnyPlayerInAnyDynamicArea(recheck = 0);
native GetPlayerDynamicAreas(playerid, areas[], maxareas = sizeof areas);
native GetPlayerNumberDynamicAreas(playerid);
native IsPointInDynamicArea(areaid, Float:x, Float:y, Float:z);
native IsPointInAnyDynamicArea(Float:x, Float:y, Float:z);
native AttachDynamicAreaToObject(areaid, objectid, type = STREAMER_OBJECT_TYPE_DYNAMIC, playerid = INVALID_PLAYER_ID);
native AttachDynamicAreaToPlayer(areaid, playerid);
native AttachDynamicAreaToVehicle(areaid, vehicleid);
forward OnPlayerEnterDynamicArea(playerid, areaid);
forward OnPlayerLeaveDynamicArea(playerid, areaid);