When I go to my guitar tech, he said that my guitar's neck is already bent. How to know it or what are the methods to see if it's already bent or not?
Well, my guitar don't really have a truss-rod. So, it can't be repaired.
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Clamp down the first and the last fret with either capos or elastic bands and measure the distance between the string and the 7th fret ideally done with a feeler gauge but a ruler is acceptable. Usually the measurement will be around .007 inches (a slight up-bow) or there will no gap (a straight neck).
However, it should be noted that if you have a significant back-bow there will also be no gap at the 7th fret. However in such a case, if you are unable see it via a glance you can use a straight edge and lay it flat on the 7th fret and you should be able to see an increasing downward angle. Very similar to the idea of running a string across your yard to see if the ground is level.
As for the other extreme ie an up-bow there should be a fairly large gap at the 7th fret.