The answer is: 'It depends.....'. Tabs in pipers is usually about 60-70% full. It should be written next to the filler neck.
The only sure fire way of knowing the difference is to fill it to tabs then fill it up. The difference will tell you what you want to know.
On the warrior full is 20 imp gals per side and tabs is 15 ig per side. The arrow may have the same tank.
Get used to looking at the tanks before and after each trip. You will get the feel for how full it is by how far above and below the tab and above and below the bottom it is. In the warrior when it touches the bottom there is usually 8-10 gallons still there.
Good luck with the Arrow. Lovely plane, be gentle with the turbo. Which one is it?