Certainly a rooster will have a big red comb and wattles, but hens who are approaching laying often do too. At 18 weeks old though it should be fairly easy to tell if they are pullets or cockerels if you are familiar with the breed. The feathering, for example will be different - roosters have pointy hackle, saddle and sickle feathers by then but pullets do not.
The best suggestion I can give you is to post up a couple of pictures - a good, close up full body profile, a clear tail picture, and a head/neck picture - and I am positive someone on here will be able to clarify it for you.
Good luck!
- Krista