If you want to determine the level of allergens you will probably need to make sure your vacuum's interior is 100% clean (without using chemicals), and then vacuum the mattress and the mattress ONLY.

Send off the "dirt" contents off to a lab that deals with that and then take the total amount of the allergens they report to you and divide it by the square feet of your mattress for an allergen concentration.

I don't know why you would want to do this though... and it will be costly.