no
It should have a portrait of Benjamin Franklin in the center of the bill. If you're referring to the watermark of Franklin, that wasn't added until series 1996.
It's still worth one dollar.
Yes, but only on modern bills. When looking at the front side of the bill, the watermark is to the right, visible when held to light. It shows President Jackson's portrait.
At least $100 US dollars (isn't it obvious?)
Any U.S. million dollar bill you might find is only a novelty and is NOT legal tender.
No.
No. Only $5 and up have watermarks, and only those issued since 1996.
There were no series letters on 1988 US $50 bills. You may be looking at the Federal Reserve District letter. Series letters, if present, are small and next to a bill's date. There's more information at the Related Question.
Ulysses S. Grant is on the $50 dollar bill. This bill weighs about 1 gram, has an identifying watermark, and is printed on cotton and linen. It was first printed in 1861.
How much is a series of 1917 act or 1863 series one dollar bill worthHow_much_is_a_series_of_1917_act_or_1863_series_one_dollar_bill_worth
Yes, most U.S. bills from 1996 on, including the $20 note, have watermarks.