How can you tell shocks are 'worn' or bad?

[Night Fury]

Could i get away with getting new shocks first (Bilsteins) and then lowering springs w/o shock extentions?

 

[Modded]

Yeah you can.

 

[HankHill]

Not to threadjack, but I just pulled off my 150K OEM Bilsteins and I'm impressed! I expected them to have no compression nor rebound, but they didn't feel all that bad! Can't wait to get the fresh set on there, though..

 

[brucebruce45]

Yea I was gonna say about 40-50k is when they need to be changed

 

[Cocotits]

Curious as I haven't ever ordered shocks before and I went to the site ajusa and they have 5100 series, mono tube, Twin tube , and regular. Search for what fits my truck and Mono tube is the only one that fits. Are mono tube my only options? And also because I'm lowered do I need shocks that are shorter then stock length, so there's no bottoming out on shocks?