There are swabs that turn pink when they contact lead paint, but if there are multiple layers they only work on the layers they can make contact with.

Lead testing is also performed using a radiation source and measuring the back-scatter (in situ testing) or by sending paint chip samples to a lab.

If you send samples be sure to go all the way through all the paint to wood to get a reliable answer.

I have a question about lead paint that might help you, snoggle:

Is lead found only in oil-based paint?

If the inside of the shed wasn't painted, use those sides of the bricks to face the inside of the beds--repaint the outsides to seal in the old paint.

Paint does not soak into bricks much so I'd say it should be okay if done that way...you could also line the beds with plastic sheeting to keep water from getting into the bricks.

if the paint is in good condition, you could conceivably paint it with an encapsulation paint (this is not normal paint), then face that towards the outside like columbusguy suggests.

if it is peeling, i wouldn't use it at all.

unfortunately, the only reliable test is to take it to a lab. there are home kits where you mail the sample out or if you are in a city, you should be able to find an environmental testing lab locally that will test for a nominal fee.