How can I check whether a struct has been instantiated?

A struct is a value type - it's never null.

You can check against default(ProportionPoint), which is the default value of the struct (e.g. zero). However, for a point, it may be that the default value - the origin - is also a "valid" value.

Instead you could use a Nullable<ProportionPoint>.

because p is struct it never be null so you should compare it to it's default value. In order to check equivalence between your value and dafault value. If you use == you will get

cannot be applied to operands of type 'ProportionPoint' and 'ProportionPoint' error

because structs do not get an implementation of == by default. so you need to overload the == and != operators in your struct like this:

public static bool operator ==(firstOperand op1,  secondOperand2 op2) 
{
    return op1.Equals(op2);
}

public static bool operator !=(firstOperand op1,  secondOperand2 op2) 
{
   return !op1.Equals(op2);
}

and then :

if (this.p == default(ProportionPoint))

another option is to use Equals directly:

f (this.p.Equals.default(ProportionPoint))

structs are value types and they can never be null contrary to reference types. You could check against the default value:

if (this.p.Equals(default(ProportionPoint)))

A struct can't be null. Its a value type, not a reference type. You need to check against the properties with the default values. Something like:

if(p.X == 0 && p.Y == 0)