How can one approximately guesstimate wave drag?
Usually wave drag $c_{d_w}$ starts becoming relevant at the drag divergence Mach number $M_{DD}$, which is defined as 

$$\frac{\partial c_{d_w}}{ \partial M}_{M=M_{DD}}=0.1 \tag{1}$$

Or in words, the Mach number at which the wave drag coefficient versus M has a gradient of 0.1

Lock$^1$ has derived an empirical formula that shows the development of $c_{d_w}$ after the critical Mach number, $M_{cr}$, which is the number at which the flow becomes locally supersonic on the wing:

$$c_{d_w} = 20(M-M_{cr})^4 \tag{2}$$

If we apply the definition of $(1)$ to $(2)$ we obtain:

$$M_{cr} = M_{DD} - \left(\frac{0.1}{80}\right)^{1/3} \tag{3}$$

As you can see, the value of $M_{DD}$, is a little bit higher than the value of $M_{cr}$. First you get local super sonic flow, but the drag does not increase rapidly, as you further increase the Mach number, the drag starts to increase significantly, and you reach $M_{DD}$. See this image, where $M_{DD}$ is denoted by $M_{CDR}$.



Source

The value of $M_{DD}$ can be estimated using:

$$ M_{DD} = \frac{K_A}{cos \Lambda} + \frac{t/c}{cos^2 \Lambda} +\frac{c_l}{cos ^3\Lambda} \tag{4}$$

With $\Lambda$ is the sweep angle, $t/c$ the relative thickness, $c_l$ the lift coefficient, and $K_A$ the airfoil technology factor (a correction factor denoting the quality of the airfoil).

So the workflow is as follows:


Estimate $M_{DD}$ using $(4)$
Derive $M_{cr}$ using $(3)$
Determine the development of $c_{d_w}$ using $(2)$.


For more info, see this link and this link

$^1$ Hilton, W.F., High Speed Aerodynamics, Longmans, Green &amp; Co., London, 1952, pp. 47-49

Note, this method is only for estimating the wave drag coefficient around the drag divergence Mach number, I'm not sure to what Mach number it provides an accurate estimate