To test injectors, pull the fuel rail with injectors attached and try to start it. See if they spray/ leak/don't fire. Since we think you have a lean code, it is possible one or more injectors may not be firing?
Here is the smoke tester I use...It requires having a compressor.
https://a.co/d/8zN4LMn
There are other cheap ones that look good with decent-ish reviews that include a compressor:
https://a.co/d/1JG2yL9
If you don't want to invest in smoke tester (mine gets used all the time), get a quote from a mechanic to do a proper smoke test - preferably from a BMW specialist- and go watch him do it to learn.