@burcidi-- Syphilis is highly infectious in the primary stage, which starts three weeks after infection with the syphilis bacteria. During this time though, the person will almost always develop a single chancre in that area.
In the case of oral syphilis, the person will have a chancre in the mouth. This is when the chances of infecting a partner during oral sex is the highest.
Unfortunately, people don't always notice the chancre because it doesn't cause any pain. So it is possible that the person wasn't aware of the chancre even though it was there. You can check oral syphilis pictures online for images of chancres if you're not sure what to look for.
The best thing to do is get to get a syphilis serology test four weeks after potential exposure to syphilis to know if there is a syphilis infection or not.