For a long while, I worked with a loss prevention department at an amusement park. Our job was to handle all the incoming cash, check it for counterfeiting, and balance tills. We would have thousands upon thousands of bills to check.
Flat out, the easiest/quickest way to check for a counterfeit these days, by hand, is to just look at the water mark, and make sure that it matches properly with whatever bill is at hand. Money markers are nice, but the latest trend in counterfeiting bills is to bleach out a $1 or $5 so that it's blank, then print a 1950s, 60s, 70s, 80s, or even early 90s series $100 or $50 onto it. These bills will pass the money marker test (because it's composition is that of a real bill), and it will look and feel darn convincing. The giveaway though is that $1 bills do not have a watermark, and $5s have a Lincoln watermark. Genuine $50s and $100s from those older series will have watermarks of Grant and Franklin respectively. If anything doesn't "line up" that bill is a fake--every time.
Now, older big bills that do not have a watermark will have one last saving grace feature to look for to spot the real deal: interwoven threads that are red and blue. Smaller bills won't have that, and therefore if anyone tried to bleach a small bill and print big money on them, you can tell by looking for the threads.
So, leave that money marker in your desk, and just hold the bill up to the sun for a second, and you'll be safe.
FYI, for anyone who wants to know, money markers are just markers with watered down iodine in them. Iodine reacts to organic compounds like wood pulp and turns brown/black on contact. Since American money is made of cotton and other things other than paper, the iodine doesn't react and turn brown. It just stays yellow.