I agree with Tom that the appropriate thrust angle should be built in . To check though once you have the motor mounted put the prop on and have a look from above with the prop set at 9 o'clock/3 o'clock. the prop should be slightly angled to the right of the plane in other words the tip on the left side should be a bit further forward than the right. Repeat the procedure from the side with the prop at 12 o'clock and 6 o'clock. In this case the top of the prop should be little further forward than the bottom. In both instances the amount is small but noticeable perhaps a few millimeters. Sorry to be a bit vague about this but ti really isa "best guess situation". In the event that the amount of down or right thrust proves to be too great or too little on test flight then washers as packing between the mount and the firewall can be used to correct. If there was no down or right thrust built into the firewall ( as determined by the checks detailed above ) then start with say 3 washers (about 2-3 millimeters total distance off the firewall ) behaind the top left engine mounting bolt and 1-2 washers behind the bottom left. the plane will fly perfectly well if you don't have this right but better if you do. Strongly suggest you have the test flight made by an experienced pilot who can advise not only on whether the thrust is correct but other issues as well. You can get caught out. For example I recently finished building a 12 foot span giant Telemaster ( from a Hobby Lobby kit) powered by a Zenoah G62 gas engine. I have been building and flying R/C models for 30 years and had thoroughly ( I thought ) checked this model over before taking it to the field. As is my usual practice I asked a trusted and experienced fellow modeller at the field to do a pre flight check and test fly it . I always find this works better for initial sorting out as I'm always too nervous. He pointed out to me that I had the ailerons reversed! A beginners mistake which would have proved disastrous. In Australia models weighing over 7kg (approx 15 lbs ) are classified as heavy models and have to be inspected and certified as safe to fly by authorised inspectors. This model had been inspected twice before going to the field by the inspector and he missed the reversed ailerons as well.
Don't assume anything check and then check again.