[Nickj]

Under load a 12v battery can dip as low as 9.5v but it will very quickly ramp back up to it's nominal or a bit better than about 12.7v
If it's cranking the motor for a few seconds then it's probably fine, a naff battery either won't even turn the lump over or will fade out after a few turns.

The nominal voltages for a battery are 2v per cell so 6 cells is 12v, in reality you find a charged battery ought to be at 2.1 a cell which gives you 12.6v. This would be a good voltage to see.
If you use a float charger then you'll often see a higher number of about 2.25 a cell giving you 13.5v which (really due to the number of people routinely using float charging) has become what people now normally expect to see. The extra you get is really just due to the battery holding a lot of surface charge on the plates which will burn off very quickly once loaded but it does give you a good kick when starting up the bike and a minute or two more cranking time on a big lumpy V.
What determines the degree of knackerment of a battery is its ability to hold charge, a real dog will get to 12.6 or more but fades out and drops volts really quickly and once the volts go down they stay down.
A good one will drop but leave it a few minutes and the volts will come back up with a small fraction of a volt drop.