The journey planner is updated and only shows trains that are running, and also the replacement buses.

These engineering works don’t affect all trains on this route. For example, you can still travel by train from Amsterdam to Leiden. But if you have to go from Amsterdam to Haarlem, then you will have to travel by bus between Amsterdam Sloterdijk and Haarlem.

From https://www.ns.nl/en/travel-information/maintenance-on-the-tracks/ 

If the planner shows trains, these trains will run but not via Haarlem (hence the warning in red).

Instead, they go via Schiphol Airport.

IF-in other cases-you need to go part-way by a replacement bus (there are most often tourcoaches, not the normal routebuses), hence seats only there are always clear big yellow signs and masts for the check/out+in beside the stop. Just follow the crowd is often also a good advice- you tipically are not the only one. The major problem on busy lines is most that if a large train arrives with a few 100 people, there have to be a lot of buses and this can lead to a crush-as anyone thinks s/he must be first.