How can I find out what's clicking on my bike? I've got a relatively new (~ 3 months old) road bike and whilst I'm riding I can hear a clicking noise coming from the bike. It seems to be when I apply pressure to the left crank. The bike is a Specialized Allez 2016, shown here.

Rotating the pedals on the cranks by hand, there doesn't appear to be any grinding from dust/grit ingress. Applying pressure to the pedals whilst the bike is stationary, there doesn't appear to any noises. There's no play in the crank arms when 'rocked' side to side. Rotating the cranks by hand whilst the bike is stationary, I can't hear the noise. I've removed by stem wiped and reinstalled along with the headset bolt, making sure not to over tighten the headset bolt. Bottle cages and frame pump bracket are secure on the bike, too.

I'm running out of ideas other than it being a bearing issue with the bottom bracket or the bearings in the pedals, but after such a short period of time, I find it hard to believe that they could have gone already - I've only done coming up to 600 miles on the bike.

I'm not sure how I can ascertain whether or not the noise if coming from the BB or pedals if I can't hear it when the pedals and/or cranks are rotated whilst stationary.

Anyone any ideas what I might have missed or more importantly how to troubleshoot further?
First of all, welcome to the "mysterious click" society!

The obvious first thing you want to check if this is pedal or not. Find a certain way to reproduce the click and reproduce it by pressing on the pedal, its furthest end, only. Then try to reproduce by pressing only crank by your foot using similar force (be careful, cranks are slippery!) and watch what happens. If click does not happen, then there's a good chance all you need is to tighten your pedal a bit.

If the click is still happening, check the screws on your stem and handlebar, saddle, also check if quick release levers are tight enough. These can click if too loose too. If you have a threaded stem, tighten the center screw - it does click if loose.

It is usually very hard to understand from where exactly the click comes. Try to apply the pressure without grabbing handlebars, try all kinds of tricks to understand what is clicking.

Carefully check out all the frame and cranks for cracks. Those can happen on new bikes too. Check your chain - may be there is a loose link?

If the click still happens, then it is a bit more mysterious than the usual causes. I have had a bike which was so mysterious so that even LBS disassembled it all and re-assembled for free - the click was gone that way. Reason still unknown.