For the most part you can't true one. They are made certain lengths, widths, cambers for given trailer needs.
There should be a divit or hole about 5/16" round for the spring bolt to rest in, in the axle.
Biggest thing (besides getting the correct length) I would be concerned with is that the axles have the same size bearings.
Like MJ says, spring center to center measurements have started being the standard, but spindle lengths can still be different, especially on the real heavy duty axles and some list hub face dimensions.