Hi John,
I hope others will chime in with better advice but my sense (having been there a couple of times and watching all the postings) is that the better bet is to perhaps park on Trilby west of Taft Hill (I don't know, maybe half a mile in?) and waiting. The falcon seems to really like perching on the tallest (brown metal) power line poles but also on lower perches either north or south of Trilby. The sightings at the landfill itself seem to have been irregular, unpredictable quick fly-bys whereas the Trilby Road sightings are of the bird perching sometimes for more than an hour. You probably also have seen the recent postings about the bird also being seen perched EAST of Taft Hill Rd or even a ways south on Taft Hill (but very hard to park along Taft Hill itself.) I think being there at first light would be good as there do seem to be early morning sightings.
The day I was there, we were able to enter the landfill itself by telling the person at the entry booth that we were "just birders" and she knew all about the rare bird and waved us on in. Once headed in (follow the big dump trucks) there should be a person in a reflective vest who you should check in with and ask where it would be OK to park. But again, at that point, you are waiting for a quick fly-by it seems.
You CAN see the landfill (with all the loafing/scavenging gulls) from Trilby but it's a long ways off. I saw the gulls all get up several times and hoped it meant the falcon was present but those were false alarms so the gulls do seem to just all get up once in a while and then settle back down all on their own.
Hope this helps.
Carl Bendorf
Longmont CO