How do you diagnose a bottom bracket issue while riding?

Not sure it can be done while riding. Problems down there can be many things, such as crank/crank bolts, chainring bolts, and pedal spindles. What symptoms are you experiencing? Perhaps the bicycle mechanics forum would be a better place for this question.

It's much easier to diagnose while off the bike. Assuming this is a cup & cone bottom bracket (rather than a sealed bearing unit), it's either too tight, too loose, and/or needs fresh grease.

1. Too loose: You can feel the cranks wobble during your pedal stroke. May also hear a knocking noise.
2. Too tight: This is easier to feel with your hands while off the bike. You will feel some friction (grittiness) when turning the crank. May also make a grinding sound while riding.
3. Needs grease: Again, very tough to tell when riding, and sometimes hard to tell when feeling the BB by hand. Usually have to open up the bottom bracket see the grease.

The other potential issue is with the crank arm. Sometime the flats on the crank arm (where it interface between with the bottom bracket spindle) becomes deformed. If this happens, it will start to loosen on the BB spindle. You can re-tighten it, but once deformed it usually means a replacement crank arm is necessary.

thank you very much! There is some (a lot) of noise when I ride. When I pedal, I herd a tick tick on my bottom stroke but when I go into a harder gear and stand up and pedal, it doesn't make noise. If I sit and spin backwards, it make the noise too. I thought it was my saddle so I tightened it and no luck.

^You're welcome. First thing I'd do is to hold the end of one of your crank arms with your fingertips while off the bike and try to move it side to side. There should be no side to side movement. (Adjusting the bottom bracket is a fine balance between tight enough to get rid of any lateral movement, and not too tight that the bearings are getting squeezed. Usually requires special tools.)

If that's not the cause of the problem, you may want to check this out: Bicycle Bike Noises, Clicks, Ticks, Creaks, Clunks, Knocks Repair by Jim Langley

That, and do the same with each pedal.

Remove the chain from the chain ring, that way you will eliminate any other potential sources of noise and ticking. Then spin the cranks without the chain to see if there are any noises or grinding sounds. Check the spindle to see how much looseness it has. What type of bottom bracket do you have ??. Is it a sealed cartridge bottom bracket or is it a cup and cone ?...If the spindle on a sealed cartridge has too much play in it or if it spins rough, then it needs to be replaced... A cup and cone can be cleaned, repacked with new grease and then adjusted and it should be ok as long as the cones and bearing races aren't worn out.