Your Experience (XP) status is shown on the bottom of your HUD. By default, the game screen resolution is 1024x728. By allowing your resolution to be set this way, you can be given a accurate reading of xp you have gained by seeing the yellow bar.
By determining how much xp you have gained in order to level, you can point your mouse cursor over the bar. The Experience yellow bar shows your current progress and once the bar is filled, you have gained a level. Gaining Experience will gradually increase as you progress towards level 90.
Once you reach level 90, you have mastered your desired profession and you will be able to master another combat profession simply by talking to a Profession councilor in Mos Eisley, Coronet, or in Theed.
As of Publish 28, Traders and Entertainers are now tagged as combat professions. However, If you mastered Commando, Bounty Hunter, Jedi, Spy, Smuggler, Officer, or Medic and wanted to respec to Trader or Entertainer - You will either be a level 5 Entertainer or Trader. Your combat experience will not transfer over to Entertainer/Trader Experience gain.
But if you want to go back to your desired combat profession from Entertainer/Trader, you will regain your level 90 experience status.
If you began as a Entertainer or Trader and mastered either one, or both, and never mastered a combat profession like Bounty Hunter, Jedi, etc. - You will be at level 5 if you respec to one.