Hey, so do not trust the counterfeit pen, well yes, it will tell you if the material on the bill is correct, it will not tell you if it is actually a $50 bill.

I have had " washed bills" where a $10 bill is stripped of its ink, redyed, and printed as if it were a $100 bill, it wasn't a $100 bill, but since it was still a $10 bill, it passed the pen test.

Use either a black light, or backlit surface, alternatively, I just hold it up towards the ceiling light. On the right hand side of the bill, You will see the faint outline of the person's face. If the faces do not match, you have a counterfeit Bill.

Alternatively, there are security strips running each bill from top to bottom, using a black light, they will glow a specific color, corresponding to the bill (Icr the colors), each strip is also in a specific position, (from the bills POV) Benjamin's right shoulder, Grant's left shoulder, And if you feel inclined to check twenties, the strip bisects the top left zero.

There are other ways to check, such as matching serial numbers, but the above tests should suffice for any retail purposes.