Does the mass of the observable universe ever change? First do we have anyway to even estimate the mass of the entire observable universe?  And then is there any data that shows mass being gained or lost?  Would we ever know if someone was playing with the til.  

Also I want to be clear that I am not talking about small masses on the outskirts of the "universe" or small discrepancies in measurement or anything of the sort.  

Note:  I would like to add that maybe we should define the observable universe as NOW (x-date) so that we aren't calculating a moving target.
Even if you're only referring the "ordinary" matter (such as stars, gas, and bicycles) and dark matter, the mass of the observable Universe does increase, not because mass is being created, but because the size of the observable Universe increases. In a billion years from now, we can see stuff that today is too far away for the light to have reached us, so its radius has increased. Since the mass $M$ equals density $\rho_\mathrm{M}$ times volume $V$, $M$ increases.

As called2voyage mentions, we have several ways of measuring the density, and we know it's close to $3\times10^{-30}\,\mathrm{g}\,\mathrm{cm}^{-3}$. The radius is $R = 4.6\times10^{28}\,\mathrm{cm}$, so the mass is
$$M = \rho_\mathrm{M} \frac{4\pi}{3}R^3 \simeq 10^{57}\,\mathrm{g},$$
or $5\times10^{23}M_\odot$ (Solar masses).

However, another factor contributes to the mass increase, namely the so-called dark energy, which is a form of energy attributed to empty space. Since the Universe expands, dark energy is being created all the time.