Deadrise is the angle the hull rises up from keel level up to where the hull side meets the bottom. (Flat bottom=0* deadrise)

Place a 6' level at "keel", and a straight edge from keel up one side of the hull bottom, protractor sitting on level where level & straight edge intersect, or use bevel square & transfer angle to protractor.

Or ... use trig using right-angle triangle measurements:

(1/2 of hull width at transom outer chine to outer chine) as side b,

and straight measurement from bottom of keel to outer chine as hypotenuse c & solve for angle B 1st which is opposite side b

Sin B = length of side b/ length of hypotenuse c .

Take that result and find it in the trig table here under the Sin column and cross reference to the angle to the left to determine that angle B.

http://www.sosmath.com/tables/trigtable/trigtable.html
Then deadrise angle A= 180* - (angle C which is 90* + angle B you just solved for).

Much simpler to figure than to explain. Yikes , that's almost the 1st time I've hauled out trig in about 40 yrs. Feel free to correct if in error.

EDIT: Whoops Still simpler to just divide side b by the hypotenuse, then check the trig table to cross reference from COSINE column to find the angle to left. (Cos A = b/c)

Or..... just look up your boat specs under deadrise if the mfgr has a website or you have the manual or brochure.

Note : Deadrise in the specifications is nearly always at the transom for comparisons unless otherwise noted, but can be calculated anywhere along keel to chine. If at the transom the angle changes keel to chine(prop pockets?), I believe the 1 straight measurement is still used to average deadrise spec. .