You mean Paris Gare du Nord?

For the trains to Brussels (Thalys), there may be checks before boarding.

In most other cases, you can just board your train.

Do you mean the NOrth station in Paris? Many, many towns have a north station, including also BRussel. Here in EUR train tix are usually checked during the ride-sometimes even more as once and certainly if a train enters another country.

There are several possible choices to cover the 250 Kms from P to B: the planners will show the superfast Thalys hi-speed, this has a fairly hefty supplmt extra to pay and that includes the mandatory Seat  REServation too. But there are other, slower ways even without any surcharge on the pass. At that Nord gare in Paris one usually has t pass gates that can be opened with barcode of that REServ-for a few days perhaps even your covid-pass will also be checked. Then you find the right car/wagon and once in, your allocated seat. In this case there may be a check or maybe not, as it has already been done and no stops till BRU.

IF you mean the short hop from Nord to Zuid Statie IN BRU city: there are 6 tracks, 3 going south and 3 going north between these, with also the Central underground station in between and there are up to 50 trains/hr between the 2-just find next one and hop on, after you have activated the trip on your pass-or wrote it in the old-style paper pass. There is also a tram/light metro between the 2, by MIVB/STIB and that is not included in pass.

Apologies, yes I mean Gare du Nord Paris. Thank you for the response, I couldn’t find anyone at the station to ask. 

Every time I have traveled with THALYS, I had to show my seat reservation just before I could board the train. In my opinion this is to tell you where your seat is. Checking the ticket always happened on the train.

In Germany they don't always check everybody before boarding but in general, my impression is that another reason for the pre-boarding checks is to keep persons with invalid (or without) tickets out.