From Fe-C phase diagram

delta ferrite exist only at high temperature, above gamma austenite

while alpha ferrite exist at room temperature

until eutectoid temp. at around 727 deg C.

However, we can stabilize the delta ferrite by alloying

so it can retain even at room temperature.

As both delta and alpha ferrite are BCC,

it is quite difficult to differentiate using some characterization technique.

However, as the delta ferrite exist above gamma austenite, 

we can expect that it will exist around and between gamma austenite structure.

While alpha ferrite will usually exist around and in-between pearlite and/or cementite.

Maybe this link can help

https://vacaero.com/information-resources/metallography-with-george-vander-voort/1061-identification-of-phases-in-stainless-steels-by-etching.html

I distinguish delta ferrite from martensite in high chromium martensitic steel by microhardness measurement. Delta ferrite and martensite significantly differ by hardness.  Also delta ferrite was free from any substructure in optic microscope.

use 20% NaOH in water at 3 V dc, 10 s, used to color both delta ferrite (tan/blue) and sigma (orange-brown). Carbides were visible but not strongly revealed.