How do you know when the strait line laser level and stud finder is on a stud

Hello. The instructions can vary from model to model, but most have a green indicator light that lights up when the unit detects a stud. There is a section in the manual (for most models) that will show/inform of the unit's individual method. Here is a link to the company's product manual support page. Click on your specific model to download the manual.

http://www.irwin.com/support-services/product-manuals

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How do you know when the strait line laser level and stud finder is on a stud