"How can i measure the pressure in the flow cell to see if i'm ok or i work near the limits?"
You would have to directly connect a pressure gauge between the column outlet and the detector's flow cell. This is impractical for HPLC analysis (due to introduced mixing and excess dwell volume, plus the cost too). Normally, a detector's flow cell has an outlet to the ATM. So only a very small amount of pressure should be present inside it. If a clog or obstruction appears anywhere along the flow path from your pump to the flow cell inlet, then you could exceed the cell's pressure rating. For some flow cell types this means the flow cell will rupture and require replacement or rebuilding (often at great cost). For some other types of flow cell's, the cells are specially designed to "leak" and relieve the pressure resulting in NO damage to the cell. The leak would be how you detect it. If you have plumbed your HPLC system with properly made and seated connectors, run clean samples, clean mobile phase, do not have precipitation of mobile phase or sample during the run, then you are unlikely to ever have a clog. Use of Buffers and/or a lack of maintenance (flushing the system out every day to remove them) are the primary causes that lead to clogged flow cells.
You also asked about having a second detector in-line to the first. Yes, if the second detector (or line leading to it) clogs, then that will result in over-pressurization upstream to the first detector. Again, the best way to minimize this is following good HPLC practices and methods. Such events are very rare and often the causes of the plugs are well known.
From a purely practical perspective, with single detector system (most common), most any reasonable flow cell rating should be fine (60 Bars is fine). It pays to look it up for each flow cell you use. Some have very low ratings (esp. RID, Fluor) and can be damaged from placing small restrictions on the outlet lines to reduce bubble formation (commonly used with some applications). For dual detectors applications OR special valve configurations, if available, using a high pressure rated cell (400/600 Bars or whatever the max limit is set to for your method) for the 1st detector (or even 1st and 2nd) can be good insurance, but not necessary for most methods.
BTW: Please do not connect your pump directly to the flow cell and just ramp up the flow rate to test its rating. If the line is restricted enough (and that is what you would be measuring, not the flow cell pressure) you may simply rupture the flow cell when it reaches its max. Not very practical unless your goal is to conduct a test to demonstrate failure of the flow cell.
