Display current routing table

route -n

Discover which interface will be used for a specific IP address

ip route get 8.8.8.8

Setting wlan interfaces to be high priority for routing than eth0, if present

sudo nano /etc/dhcpcd.conf

Add these lines to the end (or add "metric" to the existing interface definitions if you've already specified a setup for the eth or wlan interfaces)

#SET WLAN TO BE HIGHER PRIORITY THAN ETH0 FOR ROUTING IF PRESENT (lower metric takes precedence)
interface wlan1
    metric 200

interface wlan0
    metric 201

interface eth0
    metric 300