Just pull the axle. It will be obvious if it’s stripped. However, since most axles are steel and these yoyos are aluminum, the chances that it’s the axle and not the bodies is nil. Given the price of those yoyos, particularly the Seed, I’d look at getting them rethreaded.
If your axles look like they still have all of their threads, then the internal threads of the yoyo are stripped. If the axle looks like it’s threads are messed up, then the axle is stripped. Chances are, if the axle is stripped, then the internal yoyo threads aren’t in great shape either. This is because the axle is made out of steel, which is a much stronger material than the aluminium which the yoyo is made out of. So when a stripped/deformed axle is screwed into the yoyo, the steel axle will deform the softer aluminium yoyo threads.
Identifying stripped threads on an axle is pretty easy. If the threads look clean, like the axles in this picture, then the axle is alright. If there are missing threads, or if the threads are crushed or deformed, then the axle is stripped and you should buy a new axle immediately and refrain from screwing the stripped axle into the yoyo body.
most yoyos can be retaped, but it’ll cost you. I believe some yoyo modders charge about $25 to retap a yoyo. Retaping a yoyo involves cutting new, larger threads so that a new axle can be put into the yoyo. If a yoyo has an axle that is already large, then there might not be enough space to put new threads in. Putting in a new, larger, heavier axle will also change how the yoyo plays.
My advice: Move on, and buy a new yoyo. It will be much less of a hassle. If you’re very attached to those two particular yoyos, maybe consider getting them retapped.