how t otell whether the bearings on my trailer are Holden or Ford?

holden bearings are INNER BEARINGS ID:31.7MM (LM67048) OUTER BEARINGS ID:19MM (LM11949). You will have to take existing bearings off the axle to check.

Pull out the bearing and check the number stamped on the lip. Go to a bearing supplier, cheaper than buying them as Ford or Holden spares.

probally niether seeing ford holden and almost all car makers buy the bearings from timken or other bearing makers

Take it apart and take the bearings and any seals to a parts counter that handles bearings.