Use everything you can get your hands on.
You have to make an appraisal of each bit of information.
It helps to know the style of the trainer and the horse.
Before the big races you also want to know the public's opinion of any "headline works". Anything perceived as a big negative or positive should be appraised.
Public Overreactions
People criticized Larry Jones when he gave Hard Spun a perfect work before the 2007 Derby. "Clockers" said he looked terrible late in the 5 furlong work. - I watch the video of his work and he went out for 3 furlongs and then cruised out another furlong or two. He looked impressive and powerful. Considering he needed to get out of the gate in the Derby, it could only help.
People worried about a slow work from Street Sense 5furlongs in about 1:03 or so... -Nobody looks at the fractions where he finishes in 11.5 seconds and "gallops out" another in about 12!
Or Curlins winter work goes in about 1:04 and everyone is depressed...-Not only is the fact that Asmussen works his horses slow ignored, but you come to find out that it was part of a two minute lick!
The Wolfson longshot at GP Monday was a rare instance of works really tipping you off about fitness. Even then he probably was too cheap without seeing all the FTS and Layoff horses and knowing that he was the speed.