Answering myself:
In API-Explorer go to "Devices"
Click on devices/devicerecords
Run GET
Obtain the UUID of the device whose Interface of the IP address you want to know.
Put the UUID of this device into the "Container UUID"
Click GET next to physicalinteraces
Obtain the interface UUID you require then put that UUID into the filter box to the right
Click GET
Then if you assigned an IP on that interface you should see something like this in the Response Text:
{
"metadata": {
"timestamp": 1524828336620,
"domain": {
"name": "Global",
"id": "e276abec-xxx"
}
},
"links": {
"self": "https://10.1.1.2/api/fmc_config/v1/domain/e276abec-xxx/devices/devicerecords/e35fe18e-xxx/physicalinterfaces/6CB2AE1E-xxx"
},
"type": "PhysicalInterface",
"mode": "NONE",
"hardware": {
"duplex": "AUTO",
"speed": "AUTO"
},
"enabled": true,
"MTU": 1500,
"managementOnly": false,
"securityZone": {
"id": "6c1f9bce-xxx",
"type": "SecurityZone"
},
"ifname": "inside",
"fragmentReassembly": false,
"enableDNSLookup": false,
"enableAntiSpoofing": false,
"ipv4": {
"static": {
"address": "10.1.1.1",
"netmask": "24"
}
},
"ipv6": {
"enforceEUI64": false,
"enableIPV6": false,
"reachableTime": 0,
"enableRA": true,
"raLifeTime": 1800,
"raInterval": 200,
"enableAutoConfig": false,
"enableDHCPAddrConfig": false,
"enableDHCPNonAddrConfig": false,
"dadAttempts": 1,
"nsInterval": 1000
},
"name": "GigabitEthernet1/3",
"description": "Connected to server xxx",
"id": "6CB2AE1E-xxx"
}