old weed will start to get mold spots and bud rot. Look for cob web looking mold, black mold(usually in the center of the buds), powder mildew(looks sort of like thc but isn't shiny and will rub off), or bud rot(yellow and browning spots, the bud will also be very soft and start to crumble)
The best way to tell is bring a good LED light, usually ones on smart phones work pretty good. Try to inspect the bud as closely as possible under the light. It should be glistening from all the crystals and have a nice vibrant plant color to it, like dark green, purple, or light green. Watch out for yellow and brown spots, especially if it is outdoor. Smell is a huge factor too. Make sure you stick your nose in the bag and take a nice fat wiff. Your nose will get better in time. It should smell pretty skunky, maybe a little sweet. Every weed smells different, but if it smells moldy to you make sure you do a closer visual inspection. Maybe crack a nug open if its okay to do so. a lot of times mold will be on the inside.
Also, like Cryptic said, watch out for hella stems or really any seeds. Good bud won't have any seeds. Sometimes you get decent bud that got crossed and still has some seeds- it's not the end of the world but you don't want bud that is 50% seed. If you are really new it might help to look at pictures of bud online from places like the high times cup gallery, that way you have a good idea of what weed should look like visually. Just remember your bud probably won't look as good more than likely, but it should look similar.