Take the weight off the wheels and what kind of play do you have? Check top to bottom and front to rear. While one person rocks the wheel the checks for what is moving. How good is the spindle? I have seen some awful wear where the bearing ride on the bottoms of some spindles.
How about dropping a string next to each wheel and measure the difference at the top of the rim to the bottom of the rim. May not be conclusive, but it could give you a clue.
A little aside, from the picture your spring appears to have some rust color on the edges. Also the shackles seem to be pushed pretty far out. You might be in need for spring maintenance.
If you happen to take the front end apart here is how you check the axle:
Front axle straightening