Viewing seats prior to booking

I am looking to book tickets from LHR to SFO and back from LAX travelling in September.
Is there anyway of checking seat plans prior to booking to ensure we get to sit together. I am worried if I book and end up split around the aircraft. Any ideas.
Is there anyway of checking seat plans prior to booking to ensure we get to sit together. I am worried if I book and end up split around the aircraft. Any ideas.