how to check cartridges/needles
I guess I’m buying used shure m44-7 cartridges with needles. Need some help on how to check if they’re ok? I’m meeting this dude in a few days to take these carts and give him cash, just don’t want to get ripped off. What do you suggest?
No way of knowing…listen to them in use.
Then make your choice mate.
The best is to test them by listening its sound output and quality. You can also try to feel the needle’s sharpness by running your finger lightly on the diamond tip. 
Buy new needles to check the cartridges with.
^ this. Needles are relatively cheap, use the ones that come with the carts as backups if they are good, and use new ones as your regulars.
Play them and see what they sound like; there’s really no alternative. I doubt that rubbing your finger on the needle will tell you anything unless you’ve been checking needles like that for years; and you run the risk of bending the cantilever and really messing it up. If you can afford it, new tips are a good idea; I tend to change my tips once every 6 months or so anyway. They’re about $30-35 US for each tip.