I dont know much

I would like to tell you the answer, but I need to learn how to tell also. The seasoned collector will probably tell us it depends on alot of factors. Each coin has certain attributes that stand out, that are more easily worn, bold mintmarks or filled, hair boldness, stars, mottos, --- so many reasons that a coin can look very similar, yet grade higher or lower. Plus, grading has its guidelines, but it is also in the eye of the grader at that particular moment in his/her life. (like Mike Cooper made referrence to earlier). They are human, and can make human errors. Unfortunatly, the anxious coin owner must bear the brunt of that wrath if his/her coin goes to the grader who just wasnt into their job that day. I guess I could also say that it just takes practice to be able to spot what to look for on the coins you collect. If you only collect silver dollars, then you probably look at more of them than other coins, so you become more familiar with them. I'm sure a few others out there have better advice, and more knoweledge, so lets hear from them.