Most PPD slingers call their stuff "Black Henna" or "henna" and lie about what's in it.  Others have no idea what they're putting on you, they've just found a way to make easy money.  You will NOT get a straight answer just by asking!  You'll have look at the paste yourself.  Traditional safe henna paste is khaki green, greenish brown, or very dark brownish green.  Traditional henna paste smells like spinach, or you may smell fragrances like Pine, Tea Tree Oil, or Mentholatum from essential oils they're using.  PPD doesn't have a smell. 
 

2)  Ask them how long you should leave it on.

If they say to leave the paste on for less than than 1 hour, they're using PPD.  Henna doesn't work that fast.  People working with real henna will tell you to leave the paste on more than an hour, as long as you can, even overnight. 
 

3)  Ask them what color the stain will be when the paste comes off.

If they tell you the stain will be BLACK when the paste comes off and that it will stay black, they're using PPD.  Henna will leave an orange stain that will darken to red brown or dark brown, but it will NOT be black when the paste flakes off.  Ask the person to put some on their own skin.  Wait 1/2 an hour.  If there's an orange stain there, it's henna.  If there's a black stain there, it's PPD.

4)  Ask them how long the stain will last.

PPD "Black Henna" stains typically last more than a week and stay black throughout that time.  Henna stains last 1 - 3 weeks, fading to orange during that period.  There are some safe body art products, such as body paints,  that come in black, but they don't last longer than 3 days.  If it's black and long lasting, it's probably PPD "Black Henna"!

5)  Ask them what's in the mix.

A reputable henna artist will answer: henna, lemon juice, essential oils.  You'll be able to smell them.  The artist should be able to show you what they're working with.  You should see green plant powder, and safe natural materials. The PPD "Black Henna" artist will probably lie. 

6) Do you want to find an artist who does safe traditional henna?

Look at the listings on http://www.hennapage.com/henna/where/artists.html . Those artists have guaranteed that they ONLY do safe traditional henna!

Do you want a source of safe, pure henna?  See the suppliers listings at http://www.hennapage.com/henna/where/suppliers.html
Those suppliers have guaranteed that they will ONLY sell safe, pure, traditional henna products and have all the ingredients listed on their sites.

There are some black body art products that may be safe and do not contain PPD.

Ask to see the ingredient list! If the artist cannot supply a complete ingredient list, don't let them put it on your skin! 

If you don't like what you see in the ingredient list, don't put it on your skin. 

Sniff it.  If it smells vile, don't put it on your skin. 

The safer black body art products only last a few days at most.  Ask the artist how long the design will last.  If the answer is "Not more than 3 days" the body art product may be safe.