How to detect that bpftrace has attached its probes and is ready to trace?

Question

I working with bpftrace scripts attaching to hundreds of probes and sometimes it takes a couple of seconds or minutes for bpftrace to start tracing. This is not an issue if I'm providing the command via -c because the command starts when the tracer is ready. In general, however, I'd like to know if there is a way detect that the bpftrace program is ready to trace. I'd like to run my command outside of bpftrace (i.e., without using -c) but for that I need to be sure that bpftrace is ready.