How do you test tie-rod ends and ball joints to know when they need to be replaced?

slop and torn boots is what i have been told, and learned

Look at the joints themselves. If the boots are torn, replacement will very soon be required, as all of the grease has escaped, and it's likely dirt is getting into the joints themselves and grinding all the clearances much larger for you.

If you grasp the tie rod itself, see if you can move the rod by hand without the hub moving. There should be zero play. On the balljoints, jack up one wheel, grasp the wheel top and bottom, and try wiggling it. It helps a lot to have someone else observe the hub, to see if the play is in the ball joint, or just the wheel bearings. You can also try pulling the wheel off, grab the bottom of the strut, and see if you can detect any play wrt to the A-arm. There shouldn't be any at all.

If the boots are intact but look all "grainy", they're old and very brittle, and really should be replaced before they let go. Best time to catch them, in fact.

Replace them before they become two distinct and seperate parts.