I set one straight edge between the bike and sidecar. But having said that, that's where you need space to adjust things. Still so long as the straight edge is longer than the bike. If you block it up so as to be close to axle height. Then measure ( usually multiple times) till it is parallel to both the front and rear rims. The front needs moving till its dead ahead. And the rear can be out of line as well. Tyres can be lumpy and not true. As can rims. Spin to see. But you should get within a few mm easy enough I tend to clamp a bridge across from one straight edge to the other to keep them parallel also. Mine is a lot easier as I can clamp a straight edge between the bike and sidecar body onto the sidecar frame. Toe in is set at the sidecar wheel. 2c.