General appearance says a lot about a machine. Straight, undamaged panels absent of rust and scratches usually means a machine has been looked after.
Hydraulic hoses should be undamaged, leak-free with coupler-heads free of rust.
Serious rust on a machine is a good indicator of poor maintenance, lack of washing after use and damage during work.
Check for vertical play in the spreader discs. A little is OK but a lot means the supporting bearings need maintenance or replacement.
Discs and vanes should be smooth. A little wear is acceptable but the rougher the surface the less accurately the machine will perform.
Hopper grills should be straight and prevent large objects getting to the agitators underneath. Bent or twisted grills means either a large bag of fertiliser or foreign object has fallen from a height on to them.
The PTO should have a full undamaged cover and safety chain at either end. The universal joint should move freely and smoothly when handled.