Middle Pair and Low pair

When you have middle pair or low pair how do you determine when you are beat and when you are not ?

What are the percentages that the oppenent has the card that beats you when you have middle pair or low pair?

Not really about percentages imo.
You need to understand how the players at your table are playing. DO they only bet top pair and check or probe bet anything else? Are they aggro and try to push you off you tpgk with second pair? How do they bet in different positions with similar hands.
Personally if i can see a turn or river for free with middle/bottom pair i will unless i feel i can push others off of hands that are marginal like mine with check raising or simple big betting and raising. I mite also probe bet to see whats out there or maybe ill just make a nice big bet from near the button with second pair hoping no one has checked a monster looking for action.
Really, betting second/third pair is about understanding the dynamics of the table you are at. If people dont wwant to call down with anything but top pair or a good draw then you may definatly be able to steal some pots with marginal hands. In other cases you may just keep getting rumbled.