Short answer: a legitimate glass company should offer a written warranty covering both the glass itself and the installation labor, typically for at least a year, with clear terms on what's and isn't covered.
What a real warranty covers
Manufacturing defects in the glass, seal failures on double-pane units, and installation errors like improper sealing or misalignment should all be covered for a reasonable period.
Red flags
Verbal-only promises, warranties that exclude labor entirely, or a company that gets defensive when asked for warranty terms in writing are all worth taking seriously before paying anything.