Reduced Form of an econometric model, identification problem and test Looking for some help to understand the following problem and how to use the reduced form in econometrics

Consider a model for the health of an individual:
$$health = b_0 + (b_1)age + (b_2)weight + (b_3)height + (b_4)male + (b_5)work +
(b_6)exercise + u$$

assume that all variables in the equation with the exception of exercise are uncorrelated with u. 

A)Write down the reduced form for exercise, and state the conditions under which the parameters of the equation are identified.

B)How can the identification assumption in part c be tested?



Is it correct to assume:

$$exercise = b_0 + (b_1)age + (b_2)weight + (b_3)height + (b_4)male + (b_5)work +
  u$$ as the reduced form?

and is the condition for identification of parameters simply 

$E(exercise|u)=0$

and how can I test it? But moreover what is it good for?
The question doesn't make much sense to me as stated. If the problem says that exercise is endogenous (correlated with error term), you can't assume the opposite in the solution. Plus, one usually speaks about reduced vs. structural form in the context of IV estimation. If exercise is endogenous, you need an instrument for it (variable that predicts exercise, but doesn't affect health otherwise) to obtain causal effects. For example, if some people in your sample randomly won gym membership coupons, that could be a valid instrument. 

Identification assumptions would then be 


coupon really does predict exercise
coupon is orthogonal to $u$


What is called structural form would be two equations, one your original model, the other regression of exercise on coupon and other explanatory variables from the original model (the first stage). Reduced form would be when you substitute first stage into the main equation, so you regress health on age, weight, ... , work and coupon (but not exercise, as that has been substituted out). Reduced form is sometimes used for explaining properties of IV estimation, but AFAIK it's not much used in practice.