Geo-environmental/geotechnical engineer here – you can hand dig an inspection pit, as munrobiker says.
Other options include – mini digger if you think its close to surface. Any more than 1.5m/2,0m ish deep and you might struggle. GPR is possible, again if the rock head is closeish to the surface, and it can “see” through the rest of the strata.
If its deeper than a couple of meters then you’d be looking at a cable percussion drilling rig, around £800/day and a bit messy, or a CPT rig at around £1500 if the formation above the rock is soft enough to push through.
In your shoes, I’d hand dig a pit with a site spade – dont just do a single pit, do at least 2!