This guide will probably be more useful to people who are playing in higher ranks, as there's less of a chance of skill discrepancies causing outliers in statistics, if you use this guide to determine if someone in low rank matchmaking is cheating you might run into false positives in extreme cases.

There are three websites that I generally use when I want to check someone during a match:

csgostats[csgostats.gg]: This website is good for checking statistics overall in matchmaking (as it should have a significant amount of data on most players) though they recently added tracking for esea and faceit which is the single easiest way to find a linked ESEA account currently available.

faceitfinder[faceitfinder.com]: This website is useful for finding linked Faceit accounts, as csgostats will show very limited statistics (0-2 matches for most players who have played Faceit previously) and also doesn't provide a link to the account. This is also useful for seeing historical stats on client, as the aditional available information shows their peak elo and overall rating.

steamid UK[steamid.uk]: This website is only useful if you need to see someone's previous aliases, though it does have some other useful links though not generally tied to knowing if someone cheats (steamrep link to profile to check if they've scammed people before).

BONUS TOOLS:
I personally don't like legit-proof[www.legit-proof.com] because it's a bit archaic and most of the data is from 1.6/css. It can be useful if you think someone may have bought or cracked an old legit account to compare recorded names or aliases, but it's not always accurate (and some people change aliases). I would suggest only using this after the match because you might have to try multiple different versions of the player's steam ID to get it to work (same way ESEA used to be for searching steam IDs)

The most simple and sometimes overlooked tool: open your console and type status. This brings up every player in the match's steam ID, which can be useful for getting a basic idea of the age of the accounts and can also be searched on some websites (eg. legit-proof, esea) though it can be difficult to get the searches to work.

After the game you can look at stats on leetify[beta.leetify.com]:
(making an account is free and it's pretty useful for analyizing what you need to work on to get better)

Generally what you want to pay attention to is the Aim Stats tab:

Crosshair placement: depending on the map legit players can have around 4-5 degrees if they really know the map (eg. office pros), but an inexperienced waller will have closer to 2 degrees (as they can easily predict and pre-aim but also don't try to hide it).

Time to damage: of under 350ms is where you should start to be suspicious but that's not a guarantee by itself, anything lower than 250 is almost impossible for even (most/all) pro players, and if you see 0 that's a spinbot (so anything between 250 and 0 is damning on a sliding scale).

Spotted acc: is a moderately useful indicator, but only if it's above ~60% and with other stats indicating the same (if the player is consistently hitting the first shot and also hitting it in unreasonably fast time windows without adjusting their crosshair that's a good indicator).

If you see near perfect counter strafing and the other stats look fairly standard you probably just got destroyed by a good player unless the other stats all indicate impossible levels of gaming chair usage.

Generally speaking, if you feel like you're a proficient player, and haven't played more than just matchmaking you might be experiencing something called the Dunning Kruger effect when you want to call someone out for cheating who just continually destroys you. Just because you get demolished by someone who has a greater knowledge of the map or game that you're playing doesn't mean that they're cheating, and I hope that using the tools that I've detailed in the above guide can give a clearer picture to those who use it of the level that they're at. (matchmaking can be broken and give unfairly balanced matches eg. globals vs silvers on occasion)

That being said, there are a ton of cheaters in valve matchmaking so I'm sure more often than not if you review the statistics tabs of matches that you were suspicious of players in you'll find evidence of people closet cheating.

It's easy to get demotivated and to not want to play cs because there are so many cheaters and it's also natural to assume that people who are crazy skilled in matchmaking are cheating because it's so common to see cheaters. It's less frustrating to know that you're playing vs someone who is human and you can beat, instead of someone who will just change their settings to make your effort worthless.

An honest suggestion if you've read this far is to not be afraid of trying out faceit, it's free and there's a better anti-cheat (though not perfect either)