There is a technique to preparing plates. Besides the plates you need hoops, a flame, an incubator.............
Also, identifying bacteria cultures is not always that easy, and some could settle on the plates in the short amount of time that it is exposed, from your skin etc...........
I once did a 'swab' experiment around the lab and grew all sorts of nasty looking plates, which were great for scaring the office ladies into washing their hands more.
Problem: most of them were normal bacteria that are on the skin.
Would you know which were which? The 'good' bacteria from the 'pathological ones'?
I had to do micro and do experiments I found it difficult and I got the microbiologists to help me with my little experiment.
It might be a fun experiment, but how reliable? It would depend on a lot of stuff.
I would think if the gear looks lumpy, cloudy, you have any concerns, don't use it.