How can you tell if a silver certificate is an A series?

As with any US bill printed after 1928, the series letter if any will be located below or to the right of the date.

Series letters can be a bit confusing:

First, the dates on US bills don't indicate when they were printed, only the main grouping under which they were issued. Before 1974 a new date was used only when there was a design change or different printing method. If a bill used the same design the date wasn't changed. The oddest example is the 1935 series of $1 bills that was issued until the mid-1960s! Starting in 1974 a new date is used whenever a new Treasury Secretary takes office, so dates on bills change much more often than in the past.

Second, when a new date is put on bills a letter is not used at first. "A" is added only after one of the Treasury officials resigns and a new person takes office. That is, "A" is the second set of bills with that date, "B" is the third, and so on. Before 1974, the letter would be changed if either a new Treasury Secretary or Treasurer took office. Since then the date changes when a new Secretary is appointed, and a letter is added when a new Treasurer is appointed.