There might well be a clearcoat over them if the car's had paint repairs. 'Professional' painters like to throw a clearcoat over a large area to help with blending their repair, plus it keeps the old paint shiny for a while. Of course, professionals don't bother to remove the decals/pinstriping... (let me say this quite clearly though: it all depends on the price charged

)
However after 3-5 years the clearcoat starts coming off the old paint and it looks #$!@#%~ awful.
To check if you have a clearcoat, take some T-cut or rubbing compound, and rub with a white cloth on an area of the paint for a little while. If the polish residue on the cloth is a cream colour, it's a clearcoat. If it's the colour of the car, then there isn't a clearcoat (which is good news). This test is obviously a bit difficult for white or cream-coloured cars. Usually, white will be a really chalky colour, not off-white like the clearcoat residue. Also, remember that ANY metallic colour has a clearcoat as standard. The bit about paint repairs/clearcoat over a large area still holds, though.
Anyway (back on topic) if there is a clearcoat on top of the sticker, then there's almost nothing you can do - a razor blade will get the sticker and clearcoat off, but then you'll still have the outline of the sticker. Wet sand and polish to get rid of that, but there'll always be an edge somewhere, even if you 'chase' it across the panel... you'll end up having to get all the clearcoat off and do a repaint
On one car I had, I got into this situation. There was a paint-blend edge on the rear quarter as a result. I solved this problem with the addition of a bullet-hole sticker. No-one seemed to like that and I'm sure the next owner peeled it off straight away - then would have got the rude shock that it was there for a reason
Seriously, maybe think about some side graphics/stripes to hide any problems that are caused?
I'm just preparing you for the worst here - hopefully the advice from Jai (which is good advice) will sort it out

Cellulose thinners won't harm original paint - though it WILL harm any spraycan-paint repairs
If you're very, very careful, the razor-blade will be good too. To finish off the job, you'll want some T-cut or similar to restore the paint colour around the decal, otherwise I suspect the paint under the decal will show up as a very different colour.
-Alex