Firstly, we need to cover some basics. For the fuel to flow evenly the fuel tanks need to vent evenly. Air must be able to take the place of the fuel as the fuel is used. If the fuel vent system is compromised and air is not able to get into the tank, it's like putting your finger over the end of a straw as you pull it out of your drink, the fluid in the straw will stay there until you take your finger off the end, at which point the fluid will flow out of the straw.
The Cessna 150 series are prone to problems with fuel tank venting which will cause uneven flow from each tank. You need to arm yourself with the various Service Bulletins that cover this subject and make sure you have the vented fuel caps fitted. Look at the fuel caps closely and see if the rubber one way valves are intact. The one way valves let air in but don't allow the fuel out, they are normally a reddish orange colour. If one of the valves is missing it can cause the tanks to vent unevenly. The fuel tank is pressurised as described below, so the rubber valve not only prevents fuel from coming out but air as well and the tank will not pressurise properly if the valve is not intact.
In the Left wing there is a vent/pressure pipe that sticks out of the wing just behind the top end of the wing strut. It's amazing that people religiously put a cover over the pitot tube but don't put a cover over the fuel vent/pressure pipe, which is also a prime target for insects to make their nests in. This pipe can become blocked with insect larvae and cause the tanks to vent unevenly due to the uneven pressure inside the tanks. Check the 150 maintenance manual with regard to the position of the vent/pressure tube, it's fairly critical since this tube supplies positive pressure to both fuel tanks when in flight and needs to be located exactly as shown in the maintenance manual. At the other end of that tube, inside the Left tank, there is a one way valve assembly that is spring loaded shut. This is to stop fuel going down the pipe and out of the tube on the left wing, so if you see any fuel coming out of that tube, you have a problem with that valve. The valve can be stuck open or closed. To check that the valve opens you can use low pressure compressed air, blowing from the end of the pipe into the tank. WARNING, if you do this, make sure that BOTH fuel filler caps have been removed from the tank, otherwise too much pressure in the tank might rupture it. As you apply pressure, you should hear the valve pop open, if you don't then it might be stuck open already, in which case you might have noticed fuel coming out the end of the pipe. The most common reason for the valve to be stuck open is insects, and sometimes the only way to get them out is to remove the tank and the valve assembly in order to clean it, if blowing them out doesn't work. It could also be stuck closed, in which case the fuel tanks will not get pressurised and they will feed unevenly.
Lastly, if you do decide to blow back through the vent/pressure pipe, be sure to check your fuel filter after a few hours, as it may have captured a few bugs that were stuck in the pipe or valve.
As for worried about running out of fuel on the one side, you are right to be concerned. The engine will sooner gulp air than fuel, it's easier. It happened to me in a Cessna 172, I had to make a forced landing in a field as the engine stopped on short final, one tank was bone dry and the other was about 1/3rd full. fuel starvation was the cause because the vent/pressure line was blocked and the rubber valve on one of the fuel caps was missing.
