I would say you can't base it on anything, isn't that obvious? Until he plays up front for at least a full season you really can't say anything about his record. He had 29 goals in 142 games for us!
Whilst playing all over the place - when he's had a run of games up front (that brief period last season, England, a few seasons ago when he was a starter) he's generally played well and scored. For me, there's enough there to draw conclusions that - playing consistantly up front = there is a good chance he will score goals.
There was also that stat the he had one of the best goals to minutes ratio in the league last season to consider.