Mechanic said valve lifters need to be replaced

Had the valve cover gasket and the drive belt replaced on my '93 Saturn SL1. Noticed new noise when I started car and for a few miles driving. Mechanic says that valve lifters need replacing, and soon. I think they have been making noise for quite a while. How can I tell if they need replacing, and if they have been bad for quite a while, has that damaged anything else, like valves, etc. Also, I think maybe that there are two separate noises, the lifters and a new noise starting after the gasket and drive belt were replaced. What could that be? Help!!

Thank you,

Carolyn