The scopes at radio shack are $12.
You can tell when the trichs change just by looking at the plants.... but it would be hard to make that judgement on your first grow. It is possible though.
I like the couch lock, knock you off your ass high.... so I wait till things are good and amber.
You can actually see a sorta golden glow off the plants when the trichs turn.... it's not the glistening white glow they have previously.
One thing though... I use partial Metal Halide to flower, so it makes things easier to see. I think if it was pure HPS, I would use a flashlight to tell, right after or before lights on/ off. You don't want to be freaking them out shining light in the middle of their night.
I'd suggest picking up a scope if for no other reason than it's frickin cool to watch the trichs change with your own eyes.
Good luck bro.
No matter what you got some nice smoke there.