I had a similar problem when I bought my house. The tank should have a gauge on the top of it that will say how full it is; you can also unscrew this and dip a stick in to see the level if the gauge is missing/broken. I had a tank that was maybe 2/3rds full of old, old oil – at least 30 years, as my gas boiler was installed in the 80s.
I made a “free heating oil” post to craigslist and had a lot of responses, even though I was clear that it was very very old oil. The first guy that came to look at it wanted it all, so he brought some containers and a pump and pumped it all out (took a couple of trips to get it all). Then I made a “free scrap metal” post to craigslist (again, being clear that it had old oil residue inside) and again had a lot of responses. Again the first guy that came to look at it wanted it, and brought all the tools he needed to get it out.
My tank was on concrete, so there was no soil contamination to worry about. The few bits of oil that did spill during the removal, I soaked up with kitty litter. The whole process took about a week.