I don't think it's possible to visually see the wear that would start causing problems. IMO it's best to replace the needle/jet if there is many miles on the units or if you are unsure of when they were last replaced.
I have bought a set of pin gages for less than 30.00. I work on many different bikes so I have a set made of the following.
A .104, .1045, .105, .1055, .106 minis =.0002 under, .106, .106 plus=.0002 over, .1065, .107.
It's easy to tell when they are gone the pin will rock one way and not give 90 degrees to that.
With our new gas I have taken to buying .105's and reaming them to .1055. works very nice in place of a .106.
I check every new one I buy they are often no good right out of the box. It takes a good hand to ream them without chatter.