When you fly level and its crabbing that does not necessary mean its a side thrust issue. To check for side thrust what you do is as follows:

Fly away from you, into any wind and pull vertical.

A: Model continues straight up; No adjustment.
B: Model veers left; Add right thrust.
C: Model veers right; Reduce right thrust.

I have flown level on days and I can swear its yawing to one side and I will trim it with the rudder but when I pull verticals its not there. Most ARFs are built with some side thrust built into the firewall but its usually insufficient from my experience. I would say 95% of the time you need to shim it for more right thrust.