Generally, close is good enough. As Eric says, under 2:1 is probably fine.
A number of ham operators have impedance bridges or you can use a reflected
power meter. However, before cussing at your antenna, be sure the meter is
designed for the frequency range and is of good quality. The cb meters will
not give you reliable readings here.

If your swr is bad, or you are having communication problems, the connector
is more likely the problem than the antenna. I have helped out on a number
of installations and in almost every case the antenna failure is in one of
the connectors.

Remember, also, that if you place a metal object or wire in the close
proximity of the antenna, it may affect the resonance.

Colin