It's unlikely that the gate for Bucharest will be printed on the boarding pass when issued at Manchester - but you can easily check which gate your next flight departs from when you arrive at Schiphol by checking the departure monitors, or if you are a nervous flyer and need reassurance, by queueing up at one of the "Transfer" desks when you arrive (head in the general direction of baggage reclaim until you get to a concourse where you will find departure monitors and/or transfer desks - but don't, obviously, follow the signs all the way to baggage reclaim or exit. Stay airside. Don't pass passport control.)

But don't worry - Schiphol is well signposted and it is one of the easiest large airports to navigate and find your way around.

Or, more correctly, as you're transferring from non-Schengen to non-Schengen, you will not have to go through customs or immigration. (Though Romania may become a full Schengen member next year). You will have another security check at the gate, prior to boarding the flight to Bucharest.