Hmm... you know it's a fallacy just by the sound of it...
If you happen to have been to DSC, you'll realise that nurses will see MALE patients, but not female patients. The rationale for this is that venereal disease (VD) manifests clearly in males but not so in female. That is to say, women may not manifest any symptoms physically even if they do have VD.
For men, if you have gonorrhoea, you'll have the creamy discharge from your penile urethra, and if you have syphillis, you'll have the typical ulcer/rash on your penis. Women may not demonstrate this (nor smell of this). That is not to say that ALL women will not have symptoms, but a sizeable number don't. So, no, you can't tell by smell, because some don't even present with the smell or the vaginal discharge.
Secondly, and more importantly, many people just believe that gonorrhoea, chlamydia, syphillis and HIV can be transmitted sexually, HIV being the chronic disease while the rest fully treatable. Many people forget about Hepatitis B and C which are, like HIV, chronic diseases and death sentences (you're almost definitely get liver cancer with these 2 heps). And the problem is, HIV and the hepatites do not manifest anything on the genitalia.
So, bros, take precautions. There's no simple and easy way to detect someone with a disease. The risk of contracting a disease increases with the number of partner one has, and chances are, if you're sleeping with a random person, she's slept around a lot...
Cheers.