Stick your nose to the filler hole and sniff. If it smells like varnish, then the tank will need cleaned. I had pretty good luck pulling mine and pressure washing it out at the self serve car wash, but you never know. Just be sure to get all the water out when you are done. It helps to pull the sending unit so you can really get in there and blast at it. It also helps to start by pouring a bit of gas into it and letting it soak on the caked on crap in the bottom. Drain that off and dispose of it properly though.
A lot of radiator shops will dip clean them for you for cheap too. Saves effort and does a better job, but I'm cheap and my tank wasn't that bad.
You may find that it leaks once you get it cleaned up too. If it sat with some gas in it, the varnish that the gas left behind would keep it from rusting too badly, but it might have had leaks/cracks in it before it was parked too.
That all said, I've got a truck that sat for 5 years and I just dumped a couple gallons in it and fired it up. I flushed the worst of the gunk out of the lines, then put a new filter on it up by the carb before running the motor off the tank gas. Other than that though, I just ran it and it worked. I wasn't that worried about it being a daily driver right away though.
_________________
95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
