The limits are normally painted on the U/C doors or the nose leg so you can see them when tow/pushing. If not then the operator should be able to give guidance as it's his A/C you are about to break. In 30+ years I've only seen it happen once. If all else fails if it's leaving a rubber trail you are probably overdoing it, although, in my experience it's difficult to recover at that stage !!!!!!!!!!!!!!