One of the most factor is : frequency and combination.
Live game is very different to online game. (tells are very important).
Take a note in any showdown when opponent check on the river and see type of point have on river (middle value? (little-middle pair?) ? Top pair bad kicker? and so on ..
When you see how your opponent play in this situation(check on the river) u try to adjust. If opponent check middle value, try to bet big and see her reaction. and try to adjust. If opponent show often no point(two random card), we attack his check on the river ..
and so on.
If your opponent is good, have a mix value on the check when face check :
bottom value (low pair), middle value (mid pair , second pair, top pair with bad kiker ), top value (strong point like flush or straight) ... We need the right ones frequency with a right combos.
Right combos is a blocker . For example in a texture Q56 rainbow, turn 8 river 7
if opponent check , A right bluff combination is some ,KT,TJ, etc.. because block the probabily of opponent have 9T, 99 and TT (KT blokers KQ opponent combos but have a show down value..it's only an example)..
When u bluff take in mind how opponent call after check on the river and try to adjust.
Take in mind that amateur players over estimate their point and over call a lot.