irishguy28

As this appears to be a single ticket, issued on one e-ticket number, it would seem to indicate that you would benefit from "Interlining", i.e. the checking of your bag from Air Serbia to Air Canada at Heathrow.

However, I cannot guarantee that there is an interline agreement and that it would be observed for you, so you may have to collect your luggage on arrival at Heathrow (Air Serbia arrives at T4) and bring it with you to to Air Canada check-in (Air Canada uses the Star Alliance T2 at LHR).

Of course, you will have to collect your bag on arrival in the US, and clear it through customs at the point of arrival - if flying with Air Canada, this would instead be done at Toronto/Montreal or wherever you are connecting, where there is US pre-clearance in operation. [I cannot remember if pre-clearance in Canada requires you to physically bring your bag through customs, or if they just show you your bag on a screen, as happens at pre-clearance in Dublin]

Given that Air Serbia won't be operating the transatlantic leg, you can forget about getting the Air Serbia allowance of two checked bags. On multi-carrier itineraries, the "most significant carrier" rule applies, which in general, for a Europe-to-US ticket, would mean that the allowance of the airline operating the actual transatlantic sector would be the allowance that applies to the entire ticket.