Try witching for it; take two pieces of black wire 36" long and bend 6" at 90 degrees to make handles; if you know where there is a tile run step back ten feet from it; hold one wire in each hand by firmly clenching a fist around the 6 " handle with your thumbs at 12 o'clock ( the wires will have to turn up hill to cross ); pointing the wires forward at waist height, walk toward the tile line; as you stand close to or over top of the tile the wires should cross if you half the gift; if they don't try again but lessen your grip; if you can get the wires to cross, uncross them and raise them over your head: slowlly lower them and when they cross again the distance from the wires to the ground will be the aproximate depth that the tile is buried; I have witched tile drains, waterlines, electric lines and telephone lines; I am not as accurate as a locator but I can narrow down the area that the backhoe has to dig; if you lack the gift, plant alfalfa and you will find the tile in a dry year!
