How do i prove it to be a Pastel?

I'm about 90% postive that it's a Pastel, i do realize Regulars can come in a high yellow and look different, as can different morphs look like others, but this is ( in my opinion ) most definetly a Pastel, and i'd like to get it and raise it, i just don't have the money, however i've wanted a pastel for awhile, and if it is..and get it for 70 dollars because the people at Petco are idiots than i would most definetly sell some things and make it work.

How do i go about seeing if it's an actual pastel?