Chain wear, sizing and replacing

Best Practice chain wear checking;

Do not check freshly wax lubed or re waxed chains. They will be packed with wax between parts of the chain and may mask wear giving a false low wear measure. Always check before re-lube or re wax.

If on wet or other drip lubricants - be aware if lubricant on chain dirty. Some wet lubricants attract and retain a lot of contamination. This can fill in the parts between chain and give false low wear measure. You may need
to undertake some quick flush cleaning before check measure if your chain is dirty.

To check measure - - Put chain onto large chain ring, and large-ish cog on cassette. This will have the rear mech place good tension on bottom span of chain. 

Use your drop in chain checker from recommended list below. Take care of any instructions on the first insert end (ie- CC-4, pedro's - you must squeeze chain towards tool)

The drop in end is DROP IN. Force should not be used to get that end to insert into chain. Most tools can be forced into even brand new chains if one really tries. But that is not how you use, no force should be used.
Often / with some practice and experience with that tool, and checking chain from early in its life - you will be able to observe that initially the drop in end clearly hits a nice bit of roller, then less and less until that end "Drops in" past the roller for your replacement wear measure. Seeing how much roller it is catching from new - you can get a good sense re when it is starting to approach the replacement mark.

Remember to check multiple spans of chain. At least 3 is good, 5 is great, and the most worn section is your guide. Ie if tool DROPS IN on any section checked, that chain is at replacment mark.