Look up instructions for your slicer on how to get the weight (in grams) of an object before it prints.
Take your spool, and weigh it – ideally with a kitchen scale, in grams.
Go to this website and find your empty spool weight.
Subtract the spool weight from the kitchen scale weight. This is how much filament you have left.
Compare the number from your slicer to the filament you have left.
If you’d like, you can also look into getting a filament runout sensor so you don’t have to hover over your printer to catch when exactly it runs out of filament. It’ll automatically stop the print so you can replace to spool.