FOC members tend to be found chatting to one another around 25kHz up from the lower band edge. Naturally, on the LF bands you are likely to find relatively local members talking amongst themselves, while the HF bands carry significant international traffic, especially between the UK and USA/Canada.
On the "WARC" bands the 25kHz rule doesn't really work. Look around 10120 to 10125, 18080 to 18085 and 24905 to 24910. Also, 5373 on 60m and 50095 on 6m.
Roughly 25% of FOC members are in the British Isles, a further 40% in North America and the remaining are scattered among 50 (or so) countries throughout the world.
FOC members tend to have a high profile in major expeditions and it's a reasonable bet that at least one FOC member is on any major DXpedition. They'll be the ones working the CW pile-up as fast as they can go.
Most FOC members welcome calls from non-members (though please see the note in the Code of Conduct before you break into an existing QSO!).
You may be unsure whether the stations you are listening to are members of FOC. Most members are listed in the FOC listing on this web site. Also, during a QSO you might notice mention of terms such as the 'Marathons' or 'Windle Point'. As you'll read elsewhere on this site, both those terms are sure indicators of being a member of the club.
Another useful guideline is that FOC members might sign off with "161" in place of the usual "73". 161 is equal to 73+88 and is meant to convey the spirit of social element of the club by passing on regards to partners. The use of 161 is far from universal however.