A good multimeter (or a few of them ... more useful than that is one that can transfer that data back to your computer for analysis. But fancy meters often cost a bit of money.

they can’t log their readings for analysis on a computer. They’re an instantaneous instrument, just as they always have been. Lord of Hackaday [Sprite_TM] decided to hack his multimeter to ...

Most computer problems are easily traced to either ... cable and power supply (laptop power adapters can be checked with a multimeter). Also, check that the cooling fans are operating efficiently.

The Owon HDS160 is a combined multimeter and oscilloscope and comes packed with a black pouch, a manual, a specifications sheet, and leads.