How do I know when to change my crank bearings? My crank makes a bang when I pedal hard or up hill and don't know what makes the noise
To check the bearings related to the crankset, specifically, the bottom bracket bearings, one can use a few techniques. As part of my regular drive train maintenance, I check the crankset for excess lateral play by holding a crank arm while the bike is held secure and pull and push the arm several times in the sideways, perpendicular to the length of the bike, direction. I take note of any movement or sounds generated by my efforts. If excess play is noted here, it calls for tightening the crankarm fixing bolts in a system that has the spindle incorporated with the bottom bracket bearings ie: square taper or splined cartridge BB's.  If the play remains after proper torque of the fixing bolts is achieved, this is an indication that the bottom bracket itself is worn and needs replacing as a cartridge BB is not serviceable. Prior to it's removal is check that the left, adjustable cup is properly torqued and that the movement I'm sensing isn't the wiggle of the entire BB. This movement would be more up and down as opposed to lateral when a left cup is loose. Lateral play in cartridge BB's infers significant internal wear of bearings and spindle within the cartridge body.
For a system featuring external BB bearings like Shimano's Hollowtech2 or SRAM GXP, where the crank spindle is attached to the right crankam, when lateral play is detected by trying to force the crankarm in a lateral direction as above, it may be a situation where the bearing preload is too loose and needs to be adjusted properly. Depending on the system, either a left sided crankarm &quot;fixing bolt&quot; is used to load the bearings (ala' Shimano. I use quotations around the bolt because it doesn't &quot;fix&quot; or secure anything, it's a bearing preloader and the actual fixing is done with pinch bolts on the left crankarm that squeeze the splined spindle an hold it in place).  SRAM utilizes wave washers to adjust preload.  So at any rate lateral play of these systems may infer incorrect preload as opposed to excess bearing wear and needs to be addressed first.  Excess play in these systems where the preload is correct and any shims if present are in proper position is evidence of excess wear in the bottom bracket.
Another technique to assess the BB bearings is to spin the crank with chain and it's associated noise and vibration is removed from the equation.  One can simply derail the chain and get it onto the BB shell of the bike. This will allow you to turn the crankarm's while you look, listen and feel for abnormalities like grinding, excess resistance--especially when it's irregular, happens in one spot of the rotation and not the others--or the knocking or similar sound or feel associated with crank arm rotation. If you place your hand on the seat tube or down tube as close to the BB as possible without interfering with the crank rotation, one can feel the grinding or vibrations emanating from the contaminated and compromised bearings of a worn bottom bracket.  Normal findings will be silent, vibration and grinding -free, smooth rotation if the crank. The chain ring(s) should track true without lateral run out. You may be surprised, as I first was, that the spin of the crank isn't as free as, say, a wheel.  Packing with grease and preload on the bearings contribute to the lack of a comparable amount of free spin in a crankset.   Obviously this doesn't apply when there is irregular or excessive resistance to rotation, but I wanted to make note that you won't spin a chain free crankset and watch it go round and round for dozens of rotations before it pendulums to a stop.
With external bottom brackets, it's relatively easy to remove the crankset with it's attached spindle and stick a couple of fingers in the bottom bracket and turn the bearing housing back and forth. Detecting grittiness or rough rotation infers contaminated or worn bearings requiring service or replacement of the bottom bracket as most are not made to service, only replace.