This may be a simple question, but I can't seem to find a definitive answer.  I am multiclassing into Sorcerer 1st level and am already a 4th level rogue.  When I cast a cantrip, say Fire Bolt, that has damage that increases when you reach a certain level, am I casting that as a 1st level Sorcerer and therefore doing 1d10 damage, or am I casting it as a 5th level overall character and therefore doing 2d10 damage?  

I believe cantrips scale with the character's level. 

Page 4-left side

https://media.wizards.com/2015/downloads/dnd/SA_Compendium_1.0.pdf#page=4

Quote from DMAlexanderFord >>

This may be a simple question, but I can't seem to find a definitive answer.  I am multiclassing into Sorcerer 1st level and am already a 4th level rogue.  When I cast a cantrip, say Fire Bolt, that has damage that increases when you reach a certain level, am I casting that as a 1st level Sorcerer and therefore doing 1d10 damage, or am I casting it as a 5th level overall character and therefore doing 2d10 damage?  

 5th level character.  Cantrips scale on your character level, not your class level.

Thanks all!

Here's the link to the relevant rule: Multiclass Spellcaster: Spell Slots per Spell Level

Cantrips. If a cantrip of yours increases in power at higher levels, the increase is based on your total character level, not your level in a particular class, unless the spell says otherwise.