The only way to know for sure if the recruiter will redirect you to apply online is when you attend the job fair and inquire about opportunities in that company. I can see two scenarios:
1. The recruiter redirects you to the online application: If this is the case, then you are already one step ahead of other possible recruits. This gives your application better odds to be reviewed before others and possibly get an interview.
2. The recruiter does not redirect you, and asks for your contact information right there: You should then proceed to give them your information, as in this case having applied online and physically doubles your chances of being reviewed/interviewed before others.
Remember, you should still bring your contact info with you either way: In 2. you will definitely need to hand it down, and in 1. the recruiter may be willing to receive it anyways, thus further increasing your chances of being interviewed.
Either way, do mention that you have applied online, after inquiring on their jobs and company: In case 1. you could then ask them "what else can I do now I already have applied?", so there is a chance that you could speed things up now that part is done. In case it is 2. then they will see you are indeed interested in their company, which also favors you as a possible recruit.
Note: As @Weedoze suggests, it would be a wise idea to ask for the recruiter's business card.
