While stealth is cool, any Civic enthusiast would be able to determine what trim level a Civic is even without the badges. The only way you could throw someone off is if you change the rims, change the rear lip spoiler, or change the bumpers.
This is how to tell without popping the hood or looking inside:
DX & LX - Steel wheels with hub caps + lip spoiler doesn't have integrated 3rd brake light + no moonroof. DX has matte black side mirrors.
EX & EX-L - Alloy rims + lip spoiler has integrated 3rd brake light. EX-L always has moonroof / EX is standard, but might be optional.
Si - Rear lip spoiler has slits in it + Rear bumper is a dead giveaway.
There will always be exceptions, but these are the basics.