I would suggest that you have someone there at the track, to take down other dogs' trial times, just to compare, as track condition often varies from day to day, meaning times on one day can't just be compared to times on another day.

It's hard to say what would be a good time for a dog, it depends on a variety of things, such as is it their first look at the track, how much work they've had, etc.

Your trainer should be able to tell you if your dogs are any good, by watching the other dogs' trial times, rather than preempting by asking what is a good time before the dogs have even trialed.

Cheers