I'm sure this has been asked many times, but what is the easiest way to check seat / class availability for flights including code shares?

I'm looking at AC5418 and AC5210, which are UA code shares. Do I have to look on united.com and figure out what the flight is or is there an easier way?

I'd also like to know what the seat / class availability is for AC7 and AC8 which I will be connecting to/from.

I want to know how many seats are left so I can be sure to book the flight in time.