If you have a scope where the cap wont provide an adequate surface for a level, I've done the following in a pinch...
Scope resting in slightly loose rings, have your eye relief marked in advance.
Set the rifle in a solid rest outdoors or by a window where you can take "aim" at a tall house or multi-story building a decent distance away (I like at least 50 yards or so).
Get point of aim right on the corner of said building, level the rifle in the rest, then align the vertical crosshair with the edge of the building.
Sometimes it's better to hold just off the edge, leaving a sliver of light to establish parallelism.
IMO, this is probably more accurate than a very short level across a cap. Unless that building was constructed by a cross-eyed monkey, it will be within a fraction of an inch of true vertical over several stories.
Now...to avoid undue "alarm", I do this making it REALLY obvious that I'm adjusting the rifle and nothing nefariously so as not to worry the neighbors lol...