The chairman of the Federal Reserve's Board of Governors, Alan Greenspan, need not pore over page after page of arcane economic data to find signs of inflation. Beginning Apr. 15, he can look out the window at taxis passing by the Fed's headquarters at 20th and C streets in northwest Washington. Washington D.C. cabbies have just been given approval to tack a "temporary" 50 cent surcharge onto the regular fare. It's supposed to help drivers cover the recent rise in gasoline prices. Of course, Greenspan need not wait until tax day to witness inflation. A parking garage on 19th Street Northwest, about a mile from the Fed, has just raised its daily rate to $10.00 from $9.00--and that's the early-bird special for people who park their cars before 10 a.m.