Besides making sure they follow the pattern.
'5' followed by 1, 2, or 3 letters, then 5 numbers
'10' followed by 3 letters, then 5 numbers
'25' followed by 3 letters, then 5 numbers
'50' then 1 or 2, letters then 5 numbers
'100' then 1, 2, or 3 letters then 4 numbers
'500' then 1 or 2 letters then 4 numbers
'1000' then 1 or 2 letters, then 4 numbers
'2500' then 1 letter or 2 letters, then 4 numbers;
or '2500' then 2 letters over 'I' plus 5 numbers
I was warned to stay away from the specimans and anything with an X as the first letter. X i guess is a replacement note. Also to watch out for these because they could be fakes:
5ABV
50AS
500Z
1000Z
2500Z
I have seen a lot of 5ABV's in fact I have one.