How do you know what level Pokemon are?

Yes, all of your Pokemon can go up to 34.5 in that case.

There are plenty of Pokemon Go IV calculators online that will tell you the exact level - you plug the Pokemon's CP and a bunch of other data into the calculator and it spits out the Pokemon's exact stats, including its level.

There's no way to precisely determine a Pokemon's level within the game itself. The exceptions are for special situations:

Special situation 1: egg hatches - if you are at level 33, or any other level above 20, all of your hatches will be at level 20).
Special situation 2: maxed-out Pokemon. If you are at trainer level 26 and repeatedly power up a Pokemon until the "Power Up" button is disabled with the "Trainer Level is too low to Power Up" message, you can be assured that your Pokemon is now at 27.5.

Am I right in thinking that powering up until the cost equals 5000 stardust is level 29, so hitting the power up button twice more means your Pokémon is level 30?

An IV calculator that uses screenshots like Poke Genie can tell you what level a pokemon is (as long as the CP arch is not obscured).