Question XML filter [if element exists or not]

Hi,

I have a XML as source in my mapping, for a choice i have 2 elements like :
<xsd:choice>
<xsd:element name="Air" type="tns:AirType"/>
<xsd:element name="Sea" type="tns:SeaType"/>
</xsd:choice>
for the AirType i have :
<xsd:complexType name="AirType">
<xsd:complexContent>
<xsd:sequence>
<xsd:element name="AirField1" type="xsd:string"/>
<xsd:element name="AIrField2" type="xsd:string"/>
...
and for SeaType i have :
<xsd:complexType name="SeaType">
<xsd:complexContent>
<xsd:sequence>
<xsd:element name="SeaField1" type="xsd:string"/>
<xsd:element name="SeaField2" type="xsd:string"/>
...
My question is, how to test if air exists or sea (AirField1 or SeaField1), I tried to xml\inv\air = (<>) null and sometgings like this but it doesn't work.
FYI: when i'm sure that my xml is sea i can test on SeaField1 = a one particular value but with not null it doesn't work.

Thanks in advance.