Ok what I do is this.
I take the gun look at it look at the blueing I can see look down the bore (hard to see with cosmilne all over it) I then look at the stock for dents and marks like that. If it has no ###X on the stock and the bayonet isn't shiny the blueing looks good and the stock seems to have no major defects (a dent or 2 doesn't bug me cracks do) Then I just buy it clean it then look it over again. Once everything comes off it can be hard to tell if it's good or not as the cosmline can hide somethings. I just roll the dice check the serials and say good enough if it's crap I use it as a beater gun if it's not nice I buy another one and see how it comes out and use the other one as a beater..
I mean for the price of the firearm and the price for the ammo hard to screw up and even if you do just buy another one and if you shoot it out as some people say you can afford another rifle... Heck just take your average ammo usage and figure out how many rounds of some of the firearm you would shoot before it paid for the rifle and a crate of ammo sometimes it's not much. If 10 slugs out of a shotgun is 20 bucks (for good ones) that's 10 boxes to make 200 bucks for a sks...
With surplus war year rifles tend to be crappyier and more rushed but newer ones are allot nicer since they already had many rifles and only need a few more to replace the junk ones.
Make sure you take the bolt apart and clean it well that's important with the SKS don't want a slam fire or a hang fire just take the bolt and rattle it around and that's how you test them or press on it and make sure the pin isn't sticking out.
I like the pre 54 rifles because they're not cromelined and tend to be better shooters but they can be rusted allot more. Just clean it and see what it looks like and don't worry to much.
It was only 1949-VERY early 1951 that had non chrome lined barrels, everything after 1951 has chrome.
But there is no better or wrose years. Some say that the tighter tolerances of the non chrome lined barrel
can be more accurate, but it depends on the use and abuse it saw. I have found that from 1949-1952 they made more small modifications along the way, so certain parts may or may not be interchangable from year to year (for instance my 1950 gas tube fits perfect on my chinese model, but is about 3/16" longer than my 1953 gas tube) and while i would like to have one of every year, i wouldn't stress to much about the year of it.
Look to make sure the bolt slides back well, safety operates smoothly, check for a decent trigger pull, the dust cover and gas tube take down levers move easily, but not too easily (i prefer the tabbed gas tube levers, but that's just personal preference, i find them easier). Check for any rust, marks on the stock, try and find matching serial numbers if it's important to you. I also prefer the hardwood stocks to the laminate. I have picked up both and the wrist area of the laminate ones seem wider and feel akward in my hands, but again, personal preference. There is also a wide variation in wood grain and stain colour, so pick what looks good to you there.
If you plan to shoot corrosive, the chrome barrels from '51 on are probably best, but but necessarily better as long as you clean it after shooting. If you like the idea of having a return spring in the firing pin, get a '50. One warning is that they are like Lays potato chips, i bet you can't have just one!