You would Flashback. I model my flashbacks off of the ones in Dexter where you use a transition to label the flashbacks. I have also seen it labelled in the scene heading. Here's my whipped up example.
INT. OFFICE - DAY
The perfect place for a run of the mill conversation. CAPTAIN SMARTY PANTS talks to FLAPJACK MCGEE.
CAPTAIN SMARTY PANTS
I wonder if that pizza place delivers in 30 minutes
or less on the weekends.
Flapjack McGee thinks for a moment.
FLASHBACK TO:
EXT. HOUSE - DAY
Flapjack McGee beats a PIZZA GUY into a bloody pulp on the lawn.
END FLASH BACK:
INT. OFFICE - DAY
Captain Smarty Pants, still in run of the mill conversation with Flapjack McGee.
FLAPJACK MCGEE
If they don't make it on time... I just take it out
of their tip.
Onfortunately, WF is not conducive to the correct spacing... but I hope it's clear. If not, I can clarify or post up an example.