Just look at the firewall. It would be obvious if some thrust angle has been added. The mounting holes are usually off center to make up for the thrust angle and the firewall would not be perpendicular to the center-line. This allows the thrust washer / prop to be on the center line of the fuse when thrust angle has been built in. Most ARFs already add the thrust angle for you. If you ever have to add thrust angle, you can do it using washers between the engine mount and firewall as follows: With the nose pointing at you, add two washers to the upper right, one washer to the upper left, one washer to the lower right and no washers to the lower left. This gives you some down and right thrust angle. Go back and do this after the engine has been mounted because the engine and engine mounts must move together as one unit when shimmed.