"A monster with class levels always possesses treasure equal to an NPC of a level equal to the monster's final CR (as calculated in Step 3, below). To determine the value of this gear, use the value listed for a heroic NPC of that level, as listed in Table NPC Gear. Once a total GP value is determined, follow the rules for outfitting an NPC as outlined in that section. Gear should help a monster with class levels remain challenging and retain statistics close to those presented on Table: Monster Statistics by CR."
What if the monster have no extra wealth, what would be the guideline for the CR?
If you mean the monster has no equipment, like for instance, a blink dog with levels in fighter; then I would consider giving the monster even more levels or hit dice to bring his statistics up close to those presented on the table "Monster Statistics by CR".
If you want to know what the monster's CR is without the equipment then just compare its statistics to the table and make a judgement call.
The rule is saying to give the monster loot. Now if you have decided to not give the monster loot, then you have to decide the CR on your own because the rules don't cover you not giving it loot.