I am not sure of the oid value, but this may help you
http://www.juniper.net/techpubs/en_US/junos13.3/topics/reference/mibs/mib-jnx-vlan.txt
A little testing revealed the following which I hope you can use;
run show snmp mib walk jnxExVlanPortAccessMode | display xml | match oid
<oid>1.3.6.1.4.1.2636.3.40.1.5.1.7.1.5.3.521</oid>
<oid>1.3.6.1.4.1.2636.3.40.1.5.1.7.1.5.3.527</oid>
<oid>1.3.6.1.4.1.2636.3.40.1.5.1.7.1.5.4.522</oid>
<oid>1.3.6.1.4.1.2636.3.40.1.5.1.7.1.5.4.528</oid>
An interface configured in trunk mode have a value of 2
run show snmp mib walk ascii jnxExVlanPortAccessMode
jnxExVlanPortAccessMode.3.521 = 2
jnxExVlanPortAccessMode.3.527 = 1
jnxExVlanPortAccessMode.4.522 = 1
jnxExVlanPortAccessMode.4.528 = 1
Ineger value 2 = trunk
Integer valu 1 = access
Added another trunk port
run show snmp mib walk jnxExVlanPortAccessMode
jnxExVlanPortAccessMode.3.521 = 2
jnxExVlanPortAccessMode.3.527 = 1
jnxExVlanPortAccessMode.4.522 = 2
jnxExVlanPortAccessMode.4.528 = 1