I brought my bike into the lbs and the mechanic told me i should get my frame faced. is it necessary?

Depends on the frame and the bearings you're going to use and the existing level of squareness of your frame. Bearing problems arise where the frame must hold the bearing cups in alignment and the ends, the faces, are not square. This lack of squareness causes the frame to distort enough to cause the cups and cones to misalign. The two areas requiring the most attention are the bottom bracket and headset.

Head tubes with integral headsets don't need facing since the bearings are held by internal cups. The fork crown race might need facing to align the cone with the lower cup. A threaded steering tube end doesn't need facing because the torque applied by the locknut is relatively inconsequential. Head tubes using external headsets should be faced, as well as the fork crown race.

The bottom bracket shell should be faced if using a traditional cups-n-cones bearing. When using a cartridge BB bearing, facing is inconsequential since the cups only hold the cartridge in place. I've heard conflicting stories on whether shells should be faced for outboard bottom brackets, but I'd rather be safe than sorry.

Years ago my old steel bike developed an annoying and energy-sucking clunk in the BB whenever I needed to apply a lot of force--climbing hills or winding up my sprint. Interestingly, the BB shell developed curious screw-patterned distortion lines. I eventually found a mechanic who surmised, correctly, that the frame had never been faced, and performed the job and reinstalled the BB. Clunk and stress lines disappeared and I became a happy racer.

...and how am I supposed to know the answer?

Tell us WHY he told you to get your frame faced.