I would pick a representative sample, such as an entire fully developed leaf or a shoot with several completely expanded leaves, and hold it up to known examples of both species and see which one it looks like. Depending on how similar the two are you may have to study fine details. There may be an obvious difference that appears immediately or it may take some study.

Both are beautiful trees, of course. Jacaranda is the more beautiful in bloom.

Poinciana leaflets are smaller. Jacaranda leaf has an end leaflet which is larger than the other leaflets - a telltale difference.

I found the following from "Some beautiful Indian Trees" by B.Blatter and W.S. Millard, 1937

Jacaranda leaves:

Leaves are alternate or almost opposite, each with 9 to 16 pairs of pinnae, each pinnae having 14 to 24 pairs of leaflets; leaflets oblong-rhomboid, 1/5 to 1/3 in long, the end one larger.

Poinciana (aka Gul Mohur) leaves:

The leaves may attain a length of 2 ft. The leaf is composed of from 11 to 18 pinnae or minor leaves arranged in pairs along the mid-rib. The pinnae bear from 20 to 30 pairs of small oblong leaflets, each measuring 1/3 to 2/5 by 1/10 to 1/6 in.

See if the following picture helps you at all:

http://www.medamana.org/public/kerala/flora1/51.html

Slide 82 in the same slideshow has a picture of a Poinciana tree, but not a closeup.

John

I KNOW THAT JACARANDA IS PURPLE AND THE POINCIANA IS RED
AND I THOUGHT THERE WAS A YELLOW TREE SIMILAR TO THEM BUT
I CANNOT THINK OF THE NAME. CAN YOU HELP? THANK YOU.

Peltophorum (Yellow Poinciana)

Plus, there is a yellow to orange phase of regular Delonix regia (Royal Poinciana).

I love jacs, but I don't know that any tree is "the more beautiful in bloom" as reported above. Both are amazingly showy, tropical trees.