How to access peer's tip of chain? In case I'm a selfish miner and have successfully forked the blockchain, I'd like to know the state of progress of my honest nodes at any given time - how can I achieve this?

Do peers on separate blockchains exchange information about their progress through data messages?

Let's look at this scenario for a better understanding:
selfish node=s;    honest node=h

s: mine a block#1 (child of genesis block), don't report
h: mine block#2, block#3 publicly
s: mine block#4, block#5 secretly (don't report)
h: mine block#4 publicly
s: get information what block height the honest network is at!?

NOTE: 


pindexLastCommonBlock doesn't seem to be helpful since calling its
height would say that last common block is 3, but does not say what
height honest nodes are at.
setInventoryKnown in case of a fork stores all blocks ever mined.
SPV wallets typically keep track of what the best known block for each of their peers is as well, to make sure they are up to date and can assess the security of their transactions (# of confirmations). The way they do this is to start a counter initialized at the height given in the version message. And then when the node broadcasts a new inv (inventory) message for a block, get that block header, and if it builds on their previous best header, then increment the counter. With this method, you can keep a mapping of nodes to their main chain tips.

Essentially, this is just keeping track of the inv broadcasts form a peer, which is how peers publicly report new blocks.