I am cooking French dip, using a chuck roast. And I ended up having to substitute a recipe (I didn't have a can of soup so I improvised). It took a bit longer than I thought and now dinner won't be done til around 9 to 10. Is there any way I can turn the heat up do it cooks faster without it ruining my dinner?
Ok, from the clarification in the comments:
I'd say that roughly speaking, "high" cooks twice as fast as "low". This is a very rough estimate, and also depends on the slow cooker a lot (they vary in temperature...). So, each hour you spend on high is like spending two hours on low. You can use that to figure when to turn your slow cooker up so its ready on time.
Given how approximate the conversion is, you should probably plan to have it ready a bit before time, and then turn it down to "keep warm". Note that cooking will continue on "keep warm", especially as it'll take a while for the food to cool. Keep warm should be around 150°F, which will keep the food safe to eat (its out of the danger zone), but will actually continue cooking, just very slowly.
Also, you should check on when its done by visual clues (as much as possible), and (as quickly as possible) check the texture. You want to keep the lid closed as much as possible, opening it will slow it down a lot. Check no more than once per hour.
If you have a thermometer you can leave in the meat, you could use its internal temperature. My newest slow cooker has a temperature probe, but most don't, so that's probably not an option. Though depending, you may be able to stick a probe thermometer through one of the vent holes in the lid.
I have a couple slow cookers, just some fairly cheap generic brand, and they both have Low, High, and High-then-Low settings. I've seen similar settings on other slow cookers.
The one manual I can find for my cookers says: