If a dogs runs with no wasted motion, is smooth-gaited at a run with a tight tugline throughout the stride, it will likely be fast. Next I compare their fatigue level and recovery time at the end of a run. You can have dogs that try very hard to do as well as the rest, but just don't have the physical talent. I have a soft spot for dogs like this. They will give everything they've got for you.
Recovery time can be a good indication of fitness relative to the rest of the dogs in the group.
The load would be heavy enough so that they could maintain a slow lope. I would evaluate their gaits and the tugline status and look at all the factors mentioned above. If after no more than a month of this they looked good, I would start letting them run pulling light sleds distances that they could do easily and fast. I would give them the opportunity to show what they can do on their own. From there, I would go step by step, training them and culling when individuals cannot match what the rest of the team is able to do.