No always. You need to look at your spindles and figure out what the inner and outer bearing race (on the spindle side) size is and from you old hub, what the width of the bearing area is. Then you need to find hubs to match.
If you can find a brand name of the hubs or axles, typically if you buy a replacement from the same brand, odds are they will fit.
If you have a local trailer shop, take the hubs in and see if they can match them up.