It's great that you're leveling a tank too. Def helps to see what you can do better when you play both sides.

A good idea would be to learn the tank cool down icons (buffs to circumvent damage) and watch their buffs in a dungeon to see if they're using them. Some tanks ignore them completely or save them for bosses only, which a lot of cooldowns get more use in in trash packs.

Another good idea is to check their gear. Just right click and examine them and do a quick glance. If they're using cooldowns properly/gear is up to date, it's also possible their gear is broken (don't think it shows up for anyone but them tho).