I have 3-5 inch blue tilapia can anyone tell me how to determine sex of the fish?

I can't, but let's hope our friend Rainman sees this post. He can do it.

P.S. Welcome to Pond Boss.

Ken

See information in these links:
It helps with accuracy if fish are mature.
http://www.ag.auburn.edu/fish/image_gallery/details.php?image_id=1252

https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/53/

pgs 25&26 of:
http://www.cherrysnapper.com/downloads/HatcheryManual.pdf

Tilapia Sex Identification
from: http://www.da.gov.ph/tips/tilapia/tilapia.html
Sex identification of tilapia is relatively simple. The male has two openings just in front of the anal fin. The large opening is the anus and the smaller opening at the tip is the urogenital pore. The female has three openings: the anus, the genital pore, and the urinary pore. The genital papilla is usually smaller in the female. Tilapia can be sexed when it has attained the weight of 15 grams. Application of ink or dark dyes to the papillae may increase the accuracy of sexing and may allow sexing of smaller fish. By rubbing ink along the papillae of the tilapia, sexes can be readily distinguished.

Bob, did you happen to get those blues from me or someone else that got them from me? If not, they will likely be a hybridized Blue (if a Blue Tilapia at all).

Contrary to poular belief, I can't sex a tilapia accurately to save my life!!!! (and I have tried the dye too)

Rex, you guessed correctly in the larger of the 2 ponds by the house - there's Tilapia in there that are about 1/4"-3/8" long.