To gauge how well used (or abused) a piece of audio equipment is, look for tell tale scuffs, scratches around the case. Check the printed text on buttons and the remote, a heavily used item will have had this rubbed off. However some users will have used a universal remote and packed away the original one.
If you get chance to look at the internal of a piece of electronics, check the capacitors (usual upright round cylinders). Look for any bulging from sides or top and look for any leaked fluid from them. This would need replacing.
I would advice those not experienced with valve electronics to proceed with caution when it comes to buying items with valves. Partly due to the age of much of this kit; old valve kit is highly likely to have old electrolytic capacitors which definitely need replacing, due to their short life span and tendency to fail spectacularly (smokes/flames!). But also due to the fragility of valves and difficulty in sourcing replacements for some. Always be wary about opening up valve equipment, these circuit can operate at very high voltages 400-600V and often have large capacitors that can store a dangerous charge for some time after power off. Treat these with respect and if in doubt take them to a professional for service.